I'd guess that it's a gaming token, as it seems to be a very similar design to the crown and half crown. The obvious differences being the size and the metal.
But the main reason I'm thinking it's not an official coin is because "BRITTANNIARUM" has 2 T's, I thought it usually has only 1.
http://www.facebook.com/NumismaticsUK
I'm not an expert in any kind of coins, but I reckon I'm good at research and will do my best to help. Feel free to tell me my identifications/valuations/gradings are wrong. It's the only way I'll learn.