I can think of a few:
The Litten Tree - so bad the staff covered the floor in broken up cardboard boxes which soon turned into mush that was then traipsed through the rest of the pub. They then stationed a bloke with a mop in there for constant clean up. Poor bugger.
Maybush - the only carpeted pub loo I've encountered.
Ember - review says it all.
The Ship of Fools (JD Wetherspoon) - the marble floor in here is always soaked, so after a few pints it's like trying to pee on a skating rink.
Also, I dread having to use the toilet in the Euston Tap - there's normally only ever one in working order and it's never in a good state.