There are a few 'listed' cemetery's around London, the most well known is probably Highgate Cemetery where many historically famous people are buried and which you have to pay to get into. Nunhead Cemetery is free to get into and much nearer my end of town. Although there maybe aren't as many famous people buried there (none that I spotted anyway) it is more overgrown and left-to-nature allowing encouraging more wildlife, which is something I think I would really like, if I were to be buried (rather than cremated or whatever) I would love to be somewhere that nature is allowed to take over, (it is something everyone will have to consider at one time or another, afterall).
Part of the cemetery is new and being used and maintained as you would expect from a modern cemetery, but about half or more is overgrown and woody, you can ramble of the track into the ivy and find old gravestones and small monuments. I recommend wearing jeans if you're going to do this as there are a lot of brambles and I was a bit limited to where I could go because I stupidly wore leggings-ouch.
Luckily it was a good weather and it was really nice to walk around the place spotting wildlife and the occasional goth (we heard a woodpecker and saw a gothic gentleman) very nice.