The short version is that beer was brewed earlier and more broadly in the British Isles than has previously been credited. The clues have been missed largely because archaeologists have trained learning about medieval brewing and equipment, or ancient Mediterranean brewing where the grains could be dried out in the sun, but these mislead them because brewing in Northern Europe was different. It is a natural mistake. One difficulty of researching brewing is that the beer is drunk, the spent grain residue is given to the animals, and the husks, though excellent for filtration of all kinds, decompose elsewhere and leave no trace. All that is left are the structures and equipment - and much of the equipment was made of wood. The few things that could have been stone or pottery are sometimes plausibly explained by other purposes. There isn't much to look at that is definitive.
However, Merryn Dinely has uncovered some remarkable indirect evidence, and this evidence is usually a better fit for the data than what was previously supposed. Lime-plastered or beaten earth floors have been interpreted as dance floors, for example. But Ms Dinely thinks there are far too many unless dancing in smallish enclosed spaces was very common - and there is no other evidence for that. You can read more of her research and thinking at her site. I will give some summary here. Note that this is secondhand and intended to save you the trouble of going through 20 posts over at the Dinely site, and I may have not gotten it quite right in places. If you want the deeper dive, go there instead. The little bits I will add here are far less than what you will gain there.
We start with how beer is made, and how it always have been made, however much the technology now obscures some elements of the craft. The grain is grown, barley being best. The unhusked grains are allowed to germinate just a bit, to release the enzymes that start making sugars. The grains are then arrested in their growth. Often they are soaked. They are then dried, turning them constantly. They want to grow, but the turning confuses them so that they do not grow in any direction, but all over. In hot climates this can be done in the sun, but in Northern Europe this must be done inside in warmth. The grains are then put in a kiln and heated to 65 C (about 150 F). In more distant times, the grains were put in a trough with water and heated stones added. The enzymes create more sugar out of the grains. This is called malt. It is sweeter than regular flour, a pleasant taste that is still used for breakfast cereal now. Processing grain in the way goes back 20,000 or even 30,000 years. Note that this malt is not yet beer, but a product that can be eaten itself, fed to animals, or made into a sweeter bread than simple flour. It is this mix that is fermented (because sugar). Depending on the concentration of sugar, it can be made as mild as 2% alcohol or as strong as 12%. Remember that when a plant begins to germinate, the embryo is used and disappears.
Kiln fires occurred and destroyed whole batches, or even a whole year's crop. These still occurred until the 19th C, because precise control of fire in such conditions is difficult.
University of Chicago archaeologist Robert Braidwood (the supposed model for Indiana Jones's mentor in the movies) convened a food conference in 1953, and one section was "Bread or Beer?" trying to determine which was the more important driver for intentional planting of grain. Ms Dinelyn has a more encompassing answer. Malt was the driver. It is desirable because of its sweetness and can be eaten with no further preparation. But in the Younger Dryas wild grains became harder to find, and planting began.
It is not possible to have a happy accident of bread falling into water and fermenting to invent beer, despite what rumors you may have heard. But once you've got malt, accidents like that can happen.
Merryn Dinely, above, took a degree in archaeology at UManchester and became curious about the Neolithic crafts in general, such as weaving and baking, but especially brewing. So when she took a topic for her Master's in Philosophy dissertation she chose Ancient North European brewing. This was likely influenced by the fact that her husband Graham is himself a craft brewer up on Orkney. When she was reading the writeups of many digs she would find puzzling things that did not seem write, so she passed many of them along to Graham for explanation. He rapidly came to the conclusion "These people have never brewed beer. They have no idea what they are talking about." For example, many descriptions of how ancient beer was made included words like roasted, toasted, and sprouted. He knew those were incorrect enough to confuse an observer what was happening. We can now roast and toast the grain because we can more exactly control temperature, and even in medieval times they could verge into roasting and toasting if they dared. But it is very easy to kill the grain with roasting or toasting, so the ancients did not go that far. Heated would be about as far as you would want to go. And once grain is sprouted, you've gone too far and you can't make beer out of it anymore. A little germination only is the key. It matters. To dry grain in quantity while confusing it about which direction to takes large smooth floors that are not going to be rained on.
Ms Dinely kept discovering rectangular timber buildings that had burned down across the landscape and many had beaten floors or ones that had been coated and recoated with lime plaster. These floors go back much earlier than beer is believed to, but certainly malt was present in those times. They require a good deal of time and energy to make. They would be unnecessary for storing other agricultural goods in barns. More often not, the reports on contents included carbonised grain. she has obtained and tested some of these rains, and they are missing their embryos. Many archaeologists continue to prefer the explanation that these were chieftains houses that were burned upon their death. The Dinelys believe this fair screens "kiln fire of malt."
Once it is dried, the sugars of the grain are reactivated by placing them in heated water to start the sugar formation again. So what would twenty-gallon containers be placed near the hearth for, then? Not milk products, certainly. It's not enough to boil, so not water alone. Durrington Walls, which is near Stonehenge is one of the main places these large pots are found. Also, the pigs there had been fed on something sweet, likely to fatten them for sale or sacrifice. There are no high-sugar fruits in Northern Europe, and honey, have we have mentioned, is a bit scarce and precious to be given to pigs. But malt would do the trick, even residue after beer has been filtered out of it. Keep in mind that because of the dearth of high-sugar foods and the amount of labor that went into making even malt as a sweetener, even barley may have been a status crop rather than a staple crop at first. In terms of what will be visible in the archaeological record, festival sites will be more prominent than everyday. There's a lot of stuff there. They tend to persist for thousands of years rather than only hundreds, as the settlements do. And stone circles, stone rows, barrows, cursus sites, and other big-ticket structures are going to be found there more often. So the presence of fattened pigs, beer, and trade goods will be more concentrated at those sites. While we are discovering that these were more widespread than previously credited, they were still specialty sites even if every group had one or more nearby.
The Dinely's have also discovered that large rectangular timber buildings (that burned down) with lime-plastered floors are near those sites, as is large Bell Beaker pottery, and that there is not really other evidence that these were dance floors or burnt chieftains houses.
Two further notes: meadowsweet is a good preservative of ale and mead - as good as hops, and note the word "mead" right in there - and is frequently discovered in the residues inside the pots, now that we know to look there. As is true of so many things, once you think to look for it, it turns up everywhere. We now also think that even as late as the Anglo-Saxons, there was not a clear line between mead and ale, but a continuum. For those of us familiar with Beowulf and mead-halls, the scops may have given us a false idea. Poets, after all, praising. So the finer stuff, the mead, was stressed as being distributed by the minor kings in their halls, and our focus goes there.
The "land of milk and honey" captures this in a slightly different way than we are used to. Milk is a synechdoche for pasturage, and honey can refer to anything sweet. While bees and their products are certainly emphasised - again, it's the best stuff - the phrase tells us that the land is fertile for both domesticated animals and crops which can be turned into malt - and thus eventually ale.
Use all this, plus the axeheads, to modify your whole picture of Stonehenge, the granddaddy of the festival sites, as well as the thousands of more minor ones. There may indeed have been dire rituals there which we would find alarming now, but to those tribes, even human sacrifice was not incompatible with festival, lots of ale and fattened animals, or trade in goods small and large. People came from far and wide to be a Stonehenge, including from Brittany and the Low Countries and even as far away as the Alps.
My own new opinion, which I have never read elsewhere but I think follows from all we are learning recently. The Yamnaya, the Bell Beaker people, completely replaced the Anatolian Farmer populations in the British Isles while Stonehenge was still in use. They themselves used it later, though they were not the builders. We also know that those Indo-Europeans, that R1b population which now dominates u\Europe, both carried an early version of the plague and had some immunity to it. My thought is that those who came to the festivals infected the Brits on many occasions when they came to trade at festivals, and as the natives died out it became easier and easier for continental populations to move into their lands. This is not to deny their violence, as they do seem to have been more warlike than the folks in place. But that was not an either-or situation and it can be exaggerated.