I would fully recommend the Highgate Cemetery West tour - it's once a day at 2pm, costs£5 and pre-booking is pretty essential. On Saturdays it's hourly and pretty sure you just turn up and the first 12-15 get in.
It's a brilliant cemetery - not just for Horror locations but the whole atmosphere of the place is very special. I love Thomas Sayer's grave with the dog.
I'd also recommend the East Side (costs £2.50 I think) open 10 - 4ish. There you can pay your respects to Sir Ralph Richardson, Max Wall and not forgetting William "The Magic Box" Friese-Green (whose grave you can actually see through the cemetery railings from the outside).
I'm lucky enough to live within one hour's walk of Highgate Cemetery so visit a few times a year. Right now, October/November, is probably best time to go.
Website with info -
http://www.highgate-cemetery.org