His entitlement then seems to be the British War Medal and Victory Medal.
The fact he was entitled proves that he served abroad in a theatre of war with the British Army. It doesn't necessarily follow that he was a naturalized British citizen, although it's perhaps more likely than not (the Gurkhas for example are not British citizens, but an integral part of the British army).
How old was he at the time, and what's the earliest reference you have to him being in Britain?