Are you looking for hacking opportunities or for a lesson program? or both?
aside from the bhs site already mentioned, another source of listings og riding schools in London is New Rider :
http://www.newrider.co.uk/Riding_Schools/schoolsearch.php/London.html [nofollow] (The new rider online forum members are a friendly bunch too, if you're looking for a horse-specific UK forum)
the Association of British Riding Schools also have listings of riding schools approved by that association on their website
Personal notes on a few London area Stables:
When I was visiting in 2003 I went hacking in Richmond park on the evening hack at Stag Lodge stables
http://www.ridinginlondon.com/ [nofollow] Riding in Richmond Park was lovely, and the horses seemed well cared for. I haven't been back since moving here though, as on a student budget the cost of hacking was outside my means - especially as I'm still supporting my horse back in the USA!
More recently I spent a couple days seeing if it made sense to try to fit being a part time working pupil at Trent Park stables as a way to get back into the horse industry here. The working pupil thing turned out to not be a useful plan of action for my goals, but if one was looking for a lesson program Trent Park seemed to have some fairly good instructors, and decent school horses. I did not personally get a chance to go hacking there, but the path leading into the woods looked promising.
http://www.trentpark.com/TPEC/Content/About.asp [nofollow] Other than that I've been round to a few others of the London area stables either while looking at potential places to keep my horse when I ship him here, or while thinking about seeing if I could find a way to get back into working with horses. (Something I did professionally at times in the US as a sort of secondary career alongside my main one). I could tell you some impressions of those places too, but they're not based on having ridden any horses there, just on observing lessons or the conditions at the yards and how the horses looked generally.