Well, I found Fopp and CEX, so I'll give them a try. Some HMV shops outside London as well, so I'll try them as well.
Fopp is great and handily placed for Leicester Square tube and the delights of Covent Garden. The only CEX I’ve visited in London was a rather disappointing one near Tottenham Court Road. But then I don’t get down there as often as I used.
Greenwich has a couple of interesting independent shops selling used discs.
HMV is supposed to be reopening on Oxford Street in time for Christmas. Perhaps not as early as October, though.
HMV has a fairly big London store at the Westfield Shopping Centre at Shepherd's Bush. I don't remember that they had a major soundtrack section. I occasionally found something at Fopp's but it's a fairly small section. Never found a decent soundtrack at CEX, not even in their biggest store that I am aware of which is located in the shopping centre directly opposite to Shepherd's Bush Underground station. If you are looking for DVDs or blu-rays however then I can recommend these shops. I never managed to leave them emptyhanded...