MPN11
The UK highest chance of death in WW2 was at sea on the convoys I believe. Admittedly that was the Merchant Marine, but the RN was there too, taking heavy hits. Remember that when a ship goes down the casualties are a lot worse.
British vessels lost at sea in World War 2 - major warships
It is easy to forget the Pacific war also if your attention is mostly on the RAF.
Incidentally, I also believe that the chances of death in the RAF were worst on Coastal Command, then Fighter, with Bomber command the safest....