From what I've seen at beachy head there has been 100s of successful suicides, the cliffs are about 530ft tall ( ideally over 250ft for a fatal jump above water). Because it is a known suicide spot there may be volunteers looking to talk you down.
All other cliffs listed are over 400ft tall so technically should also be high enough. Although nothing is guaranteed

I'm also considering some of these spots myself and looking into them more, but ideally I would want to jump at night but most of the spots require a bit of travelling or little hike which is a little daunting for me 21F