In the attached copy of your picture I have arrowed a projectile that looks like a 15mm BESA. It has the distinctive double driving band formed in the envelope. Can you confirm that it is about 15mm in diameter please?
If so, it is another odd find as the 15mm BESA was only used in a few armoured vehicles and not by the RAF AFAIK. Picture of AP and drill round attached. (The item on the right is an armourer's headspace gauge for the 15mm BESA)
Sorry to be pedantic, but I should also point out that live ammunition, no matter how relic, is still live ammunition and requires a Firearms Certificate to possess.