Maybe fully disassembled modelguns/kits from U.K. would make it through customs?
And I thought U.S. laws were strictest but far from it. In most states any replica weapon will go through as long as there is a small orange tip or paint on the end of the slide/barrel. But I'm pretty sure even if it doesn't but the gun is visibly a replica, either airsoft due to no firing pin and obvious fake magazine or something like a plastic model gun, it would go through. If the modelgun were full metal it would be extremely hard to tell from a real weapon, besides maybe the lack of serial number so I don't know what customs would do. (anyway, someone who wanted to obtain a real firearm illegally would certainly not have it mailed to them lol)