I guess it depends what 'end' of London you're in. If you're in the South, then Biggin is an obvious choice - it's open 'til 9pm weekdays, so with these dark evenings coming fast upon us, it's plenty late enough. If in the East then I can second Keef's words that Southend is excellent. Don't know about the clubs, but each time I've flown into the airport, the boys and gals down there all appear professional and extremely friendly.
One thing, however, as the 'Summer Season' for flying clubs draws to a close, they're more keen for your money than usual so it's always worth trying to negotiate some sort of bulk booking discount even though you’re not doing a complete Night Package.
Good luck,
G