There are proof houses in both Birmingham and London. They charge about 11-15 pounds per barrel. The greatest cost is shipping and paperwork if they do not accompany you to that point. If you have to send them, expect a cost of about $300.

First the gun will undergo visual proof. If any barrel dents, pits or looseness is discovered, the gun fails visual proof and the gun must be fixed. If the gun passes visual proof, it goes on to firing proof. IMHO, anyone who tries to do this themselves is a complete idiot.