Thump, I know East Finchley quite well and used to live in and around Golders Green for more than 10 years until I moved sarff, I don't think it is any less safe in SE23 at all, plus you will get quite a lot more value for your property money down here.

I regularly use the trains a lot later than you are planning to, often come home after 10pm or even on the last train past midnight if I'm out with friends, and I have never felt threatened or unsafe. (getting off at HOP, but don't think FH should be too different). At 7.30pm you would still get a lot of people returning from offices in the West End and City, Canary Wharf workers tend to work even later of course and now also use the ELL, and you will find many people are still on the trains until at least 10.30pm.

PS. If you miss the Heath, try the top of Horniman gardens - beautiful views that will give Parliament Hill a run for its money! :-)