You would think that the best beer I could get in London would be pulled in a pub. I had a few of those on our trip, but I can't say I loved them. The whole pub thing was high on experience; very high. But the beers were just fine. They had less hops and were typically flatter than I prefer. People usually complain about the temperature of the beer. I find the temperature works well for the style.
I got my best beer in London at The Riding House Cafe in a bottle. The beer was Purity Brewing Company's Pure UBU Amber Ale. This is the best beer I've had in a long while. It was full-bodied like an American micro-brew, and maintained a perfect hop balance. I don't think I can get it in the states, but I'll be keeping an eye out. If you see it, buy it.