I went for a session last night. I'm not in London so can't help you with that part.
The reason they reccommend so many treatments isbecause the hair grows in 3 stages and the laser will only work at one stage (I don't remember which). Not even the best doctors in the world could tell you which stage the hair is at, so you just have to keep at it until all the hairs are captured.
With regard to the procedure itself, there's nothing to it nothing to be scared of. The beautician will use something that liig a bit like a rectangular shower head, when she presses the button on this thing, it will emit high heatfor a millisecond. It's not long enough for you to even think "oh, that was hot".
I promise it's not uncomfortable and I can doubly promise it's worth every single penny.