the answer is one! F-4 v Jaguar in 2 ATAF and yes, we've all heard it before and we all know how it came about!
Actually I think it's a few more depending on how you count them. Ignoring all the ones (6) by RAF pilots in SHARs during the Falklands & those flying Sabres in Korea (6) with USAF, I believe there were a couple of Egyptians around 1948 (confirmed) plus a couple of Israelis (unconfirmed) & possibly a few more including a C130 by a Javelin during the Indonesian conflict that was too politcally sensitive to confirm. There was also an Iraqi Mig 29 believed destroyed whilst taking off during GW1 by a Tornado & JP233 although as it was still on the runway it's not really an air-to-air kill!
There was also a Harrier shot down (deliberately) by a Lightning to prevent it crashing uncontrolled after the pilot had ejected.
Not that it matters one jot as you can't measure the worth of the entire RAF by such a simple measure although there are plenty of simpletons who try!