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.