It's not really London (though only an hour on the train...), but Malmaison in Oxford certainly counts as interesting - it's in the old prison! You'll be glad to know that the rooms are made up of several cells knocked together, so they're quite spacious, but the coridoors (such that they are) are the original Victorian prison style. It's also attached to the old city castle and mound, which gives some great views.

I've only ever been there for dinner and drinks, but at least some of the crowd fits what you want (the rest seem to be slightly older in very nice cars).