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Read the thread October 2010I've used The Gallery at The Hospital club in Covent Garden for that sort of thing before, although being a gallery it's a blank canvas and you have to hire in furniture etc (I normally use Great Hire) – but they have an in-house team that could probably do all that for you.
If it was a day time event I'd definitely recommend the Sketch gallery in Conduit Street, but because you've mentioned about cocktails afterwards I think the cost to go into the evening would be prohibitive.
If you're willing to move out slightly, Bermondsey Square Hotel's events suite would work though, and it also has a good adjoining terrace if the weather's half decent. More