Tapas are fun. I think it is always nice to try lots of different portions, specially when sharing with the rest of you party. I have two favorites in London (so far). One is Polpo (Italian tapas) and the other one is Casa Brindisa (Spanish tapas). Both serve amazing flavors and it is only a matter of deciding the country you desire most, Italy or Spain.
I prefer the Polpo in Soho (just a quick 5 minutes from Piccadilly Circus), although they have expanded to other locations in town like Notting Hill and Covent Garden. The ambience is always lively, service is very friendly and the selection of tapas, well, I would try anything from their menu at least once.
Casa Brindisa is a great spot to have a late lunch after a visit to the museum area of South Kensington. Every tapa I have tasted there was amazing, so be aware you can spend more money than originally planned. They also have locations in Soho, London Bridge and Shoreditch I still have to visit.
Polpo Soho > 41 Beak Street, W1F 9SB
Casa Brindisa South Kensington > 7-9 Exhibition Road, SW7 2HE