Is just finding a nice quiet pub/tavern as a tourist to have a nice quiet beer undesired in London?

In some places yes, in others no. Again, there are thousands of pubs. Some are run by large corporations/groups like Greene King or Mitchell and Butler, some are tied to a specific brewery (who will control which beers are on offer and the food menu, and the technology platforms for order/pay at table). Some are independent/free house - which are generally struggling more as they don't have the same economies of scale, so if you have a choice, do choose an independent. I personally would also generally avoid Wetherspoons as they're not what I'd call the classic British pub, although they are popular in the same way that populism dominates politics. The company has become massively successful by offering cheap and cheerful food and booze, marketed to the masses and has a very good order - at - table app. They tend to have large locations (and there's a popular meme about how the toilets always seem to be miles away). The clientele can be, erm, interesting, and it's run by an avid Brexiteer who then realised that over 50% of his staff were EU nationals. The resulting wage inflation has contributed to other current difficulties across the whole hospitality industry.