I have, I think, a very high standard for pizza. Pizza Hut (in the US) used to be just tolerable, but at some point they started to deep fry their crusts. Domino's was the worst pizza that you could get delivered before Pizza Hut made that switch.
I'm most partial to NY style pizza and old-California style pizza. My favourite pizzerias in California are Arinell's (NY style), Pizza My Heart (old Ca style) and Woodstock's (Woodstock's style).
That much established, so far nearly all the pizza I've seen and tasted in a short week and a half in London has been about the same low quality as frozen pizza (which I simply won't eat anymore).
The one exception that I've found is Mulberry Street in Bayswater, a NY style pizzeria. Their slices are pretty spot-on, with the one caveat that mozzarella cheese here is *different*. It's more gooey melted, and kind of resembles "soy cheese" in flavour and texture. Apart from that, Mulberry's is pretty darn good.