In reply to Pids:
Highbury Fields is a disaster if you don't like laps - it's 5 times around the same loop! Finsbury is also a couple of laps from memory but is a much nicer run in a much nicer park. If you are thinking to head to a climbing wall afterwards you can also take the short walk to the Castle from there.
I've not done Hampstead but understand it is quite hilly.
If you want to make the effort, the original Park Run at Bushy Park is a tube and train ride away, but is really nice.