If you're looking for NY-style pizza in London you won't find it -- at least I haven't and I've looked. But you can find good, thin-crust, brick oven pizza. My favorite is Pappagone's, a few doors down from La Porchetta on Stroud Green Road (which is also pretty decent).
I used to think PizzaExpress was a good choice but I agree with Vicky, the pizzas have gotten smaller and not as good as they used to be.
There's a place in Brixton Market called Franco Manca, that's supposed to be terrific. I haven't been myself (they're only open for lunch) but really want to try