London crowds can be mixed, mostly find them to be ok. Sure it isn't like a Spanish or Italian crowd and can at times be a little too reserved (when it comes to pop shows anyway - go see the likes of Kasabian and a whole different ball game) but mostly will respond to a good show. Compared to other places I've been in the UK (Brighton, Bournemouth, Birmingham, Cardiff, Glasgow or an utter morgue at a show in Cambridge a couple of weeks back) London is absolutely rocking.
Perception of an audience is all driven by who is directly around you. To my mind the first night was tepid (probably lots of industry / guests in), the other nights all seemed quote buoyant. All 4 however had a better crowd than either show in Birmingham (for my money).