Not in my observation. Of course, Stella is everywhere, and Leffe is also reasonably popular. Some of " the proper ones" are also reasonably common.

Budweiser is fairly easy to find and does indeed have a local following. It probably would be safe to say, however, that Bud individually outsells any single Belgian brand except Stella. I used to work for a UK company, and my boss there was a regular Bud drinker. As has been mentioned, Miller Lite, and MGD can also be found, along with Bud Lite...which for some reason seems to be more widely available outside of London. I haven't yet personally seen Coors Lite in England, although the stuff is all over Dublin.

Sam Adams is the only other American brew that's made any noticable headway. You can sometimes find it on draft in London pubs (Not very often, however).

My daughter lives and works in London. She and her (British) boyfriend are fond of Sierra Nevada and Fat Tire. Generally, these two are unavailable. However, there's a beer vendor in Borough market that gets in a small supply of one or the other from time to time. So a shopping trip for breads, cheeses, produce, etc. sometimes ends up with a nice little bonus.