I think British Law makes it illegal to counterfeit any "protected coins" meaning any coin which is customarily used as money in any country. In addition the following are also protected:
Sovereign, Half sovereign, Krugerrand, any coin denominated as a fraction of a Krugerrand, Maria Theresia thaler bearing the date of 1780 and any euro coin.
I think it might therefore not illegal to make replica of coins not included in the above. However, attempting to sell them without clearing stating they are replicas would certainly be treated as fraud, or misrepresentation of goods which would be illegal of course.