Denham? Elstree? Wycombe Airpark?
I don't know the airfields that side of London, so can't advise. Being near Heathrow does inevitably mean you are going to have to travel some distance to a GA airfield.
Have you found the UKGA website? It has an interactive map of the UKs main airfields. You may need to sign up to get access.
Good luck.