Not sure where in London you are, but the Wimbledon Wine Cellar do some good deals on Champagne, especially if you buy by the case (which it sounds like you might need to for a party). They do a St. Evremond which is the second growth of Taittinger and simply lovely - think it'll be around £20ish a bottle when you buy by the case. Otherwise they've got all the big names. Of those, I concur with what's been said, Pol Roger, Ruinart or Nyetimber are my favourites, with Taittinger and Ayala very lovely too.