I can recommend Be@One in Balham (http://www.beatone.co.uk). It's a cozy little place with a bit of character. It is a chain unfortunately, but from what I can tell it's naturally grown up from one store and been a bit of a success. I'm sure all the instances of this bar are equally enjoyable. The Balham one is pretty busy on a Friday/Saturday night, as it's not a long walk to other Balham nightspots and a short tube or bus journey from Clapham. about 10 years ago
The Player in Soho
The bar in Cocoon on Regent Street does an AMAZING Passion Fruit & Cinnamon cocktail which I've never found anywhere else.
Lounge Lover in East London is also very good.
Something a bit more hectic would be the Moose Bar opposite Selfridges, it has a club downstairs which is busy every night of the week. about 10 years ago
For cocktails try this link http://www.viewlondon.co.uk/pubsandbars/cocktail-bars-in-london-recommended-london-51.html about 10 years ago