I think one of the best-kept secrets regarding decent pizza in London is Strada.

http://www.strada.co.uk/

The pizzas are far bigger than Pizza Express (but the same cost). They are everywhere, including right south of Piccadilly Circus, and on the South Bank just downstream from Hungerford Bridge.

They also have very decent Italian food for non-pizza-eaters.

Here is their menu:

http://www.strada.co.uk/menu.html