I started quite a long reply to this thread ages ago and lost it, but here's a shorter version :
I think whatever styles of beer Tolkien had imagined when writing the books would be the styles that would be commonplace in the pubs he frequented. Those would be milds, pale ales, and bitter
This needs saying over and over. As an Oxford academic, Tolkien would likely have spent a lot of time drinking in pubs, but wouldn't have known as much about beer history as we do now. He would have assumed that they would have been drinking the kind of beer he was drinking in the 1930s and 1940s when writing the Hobbit and LOTR respectively. The Oxford scene would have been dominated by Morrells and Morland, although the Eagle & Child seems to have been run by Halls (taken over by Allsopps in 1928 who then merged with Ind Coope in 1934) and Lamb and Flag seems to have been controlled by Ushers of Trowbridge.
As it happens, Ron Pattinson has found
a price list for Flower's pubs in 1948, and matched it up with a brewing record from 1955 ( LOTR was published 1954-55). Flowers were only 35 miles away in Stratford and I think you can assume that their range was typical of what you would have found in pubs in Oxford at the time. There were two cask beers - a bitter and a mild, and the same in bottles plus a stout and an "Gold Top" IPA at more than double the price of bottles of the bitter.
Reading across to the brewing record, by 1955 the bitter (labelled as an IPA but only in the same sense as Greene King's IPA) was 1.034 OG and 3.3%, the mild was 1.032 and 2.96% - and they may well have been a bit stronger in 1955 than they were in 1948. Ron reckons the 1948 stout was weaker than the 1.040 3.35% stout of 1955, he thinks the Gold Top may have been the OB or Green Label which were both under 4.3% - so modern best bitter strength. If you want a recipe, take
this modern clone of Morrell's Varsity Bitter and dilute it a bit, maybe replace some of the malt with adjuncts. Or Ron has a
1959 recipe for an Usher's IPA (effectively a 4.1% best bitter) which Tolkien might have had on special occasions at the Lamb & Flag.
Note the absence of barleywines and ESBs -
we don't really drink strong bitter in Britain, ESB is just not a thing. Yes you get actual Fuller's ESB and Young's Special in their tied pubs in London, but in general you don't see much cask beer over 4.5%, certainly not outside city centres.
The Hobbit is about a romanticised version of the bourgeosie of rural England before World War I, it's essentially Jane Austen for boys. So if Tolkien had been a beer historian the Hobbits would actually have been drinking what English gentry of the late 19th century would have been drinking - small casks of AK as the house beer, and bottles of IPA for special occasions. See
this thread for AK recipes - essentially Chevallier or Plumage Archer with 10-15% of invert and optionally 5% flaked maize, to 1.045-1.050 with 40-45 IBU of English hops (or Styrians, Willamette etc) across 60 min, 20 min and dry hop. At least it would have been that kind of strength pre-WWI, it dropped to 1.030-1.035 afterwards.
Porter had pretty much died out by Tokien's time, certainly in the provinces, but would have been half-remembered as an old-fashioned beer that one's grandfather drank in the same way as mild is now. Frankly the
current Fuller's recipe is hard to beat but
Ron has plenty of historical recipes to play with - you can mess about with aging some with Brett-C and then blending it with freshly-brewed beer if you want.
Personally I wouldn't take the reference to "the beer of 1420 malt" too literally, I'd read 1420 as 1920, representing the relative calm that came after all the traumas of WWI and the flu pandemic.