As for your topic, Trocadero in Leicester Square and Casino in Goodge Street are the only decent ones left. As of right now, nowhere in London has a SFIV cabinet (apart from Capcom’s offices :p)
Trocadero has Tekken 6, Virtua Fighter 5, 3rd Strike, Super Turbo, Alpha 3 and other stuff on really shitty cabinets. 3rd Strike and Tekken 6 get played the most there. VF5 is set to 6 credits for 1 (!)
Casino has 3rd Strike, CvS2, Marvel vs Capcom 2, Alpha 3, Tekken 5, KOF 98 Rematch, KOF 2002 and other stuff but the sticks and buttons are always in such poor condition, no-one really bothers going there anymore. The light kick button on CvS2 was still broken after THREE MONTHS the last time I went there, which was ages ago. It’s probably still broken now. That place has more moths than regular players.
Might be worth signing up to NeoEmpire.com if you’re after competition, that’s where most of the UK players post.