Sounds like a practise round or an inert replica round.

Does it not say 'ARDEN' on it as thats a type of large calibre machine gun/cannon used on some aircraft and small forward recce vehicles although Ive never heard of a 25mm round-I thought it was Oerlikon that was 25mm.....

Have a look here mate....

http://www.antiquearmoury.net/90265/...p=3&cat=215164

That should give you an idea

Edit: Ive just looked at your picture....its definately a dummy inert round as you dont get S/S casings like that.

Worth about 8-10 quid.