1950s to 1960s - the age when the stereotype of British ale as thin, weak, and watery was forged... in the post-war period, taxes and rationing meant that recipes featured fewer and cheaper hops, increased use of sugars* and adjuncts like grits in place of (relatively expensive) barley, and lower ABVs.
Your pub would have a couple of beers on cask: mild, bitter, and, if they're lucky, a porter or stout. None would likely be above 5% ABV, most 4% or lower. The mild would be cheapest and lowest in strength, relatively dark in colour, though not as dark as the porter/stout. This is your everyday beer.
The bitter would be more expensive and more heavily hopped, though nowhere near either today's or pre-war standards - some might remember this and complain about it. Usually the bitter would be designated a "best bitter", historically to distinguish it from other varieties in the brewer's range, but by this point most small breweries stuck to a few beers. So if you want bitter, you ask for a pint of best, and the lads at the bar raise their eyebrows at you being fancy.
City pubs would likely also have a lager, British brewed rather than import, though using imported (likely Czech) hops. Lager was particularly popular in London. Compared to today's standard lagers, it would be lower-carbonation and not served anywhere near as cold - so more like a very pale ale (which, curiously, is much closer to the traditional lagers I've drunk in Germany).
In any pub, the beer runs the risk of being kept poorly and going off (sometimes called "stale"). Vast amounts of money have been spent since the 1960s on improving serving technology to help with this, but back in those days it was largely the skill of the publican that kept the beer drinkable. You might choose a pub based on how well they kept their beer. Badly kept beer can become either oxidised (sweet sherry and cardboard-y kind of flavours), go sour from wild yeast (slightly tart/puckering, some farmyard-like/sweaty aromas and flavours), or, in the worst case, become infected (anything from vinegary to metallic flavours, none subtle or able to drink around - you'd know the second you poured it, let alone drank the stuff).
Beer historians like Ron Pattinson, Pete Brown, and Boak & Bailey have excellent resources on British pubs and beers in this period, including photos, recipes, brewing records, and interviews.
*: sugars have long been used in British beer for flavour, so it's not as simple as saying "sugar is a cheap substitute for barley", but that's definitely true in some cases.
I'm trying to hew to this Earth as much as I can. Thus, no, potatoes and tomatoes won't be native to Europe in my world.
