I've just been out to check a cliff that's about 150ft, with concrete at the bottom. I think that is high enough. Probably not if you would be landing in water (depending on how deep the water is). I had been thinking about a 300ft cliff, but it's a long train journey, followed by a 1 hour walk to get there. I think I would have had the courage to jump, but it's difficult to tell.
Alternatively, I know of a good beach where I could walk in to the sea and drown (and hopefully be swept away). I thought I could do it when the tide is going out at night. But it's three hours away and having done some more research, it seems that it might be popular with people fishing, especially at night. Aarrgh why is it so difficult to ctb?!