There's Cambridge Coins that isn't in Cambridge...or Cambridgeshire... But it is close, just over in Bedfordshire. Then there's Gannochy Coins, but I wouldn't recommend them as the two times I've been in there I've been treated rather rudely - it's pretty central in Cambridge, not far from Sidney Street Sainsbury's in the back corner of a cycle shop. There's also Cambridgeshire Coins which I've no dealings with so do not know what they're like.