A working replica is considered a firearm so needs either a shotgun certificate or firearms certificate depending on what it is.

An original antique can be held without a licence as long as its not being used. As soon as you want to use you need to get it on the licence and then lock it in a cabinet. The security requirements for antiques that are on ticket are so bizaare you can't but laugh. You can have a matching set of antique pistols, one can be dangling on the front-door handle and the other identical one must be kept locked up - purely based on what you intend to use it for.