This may seem off topic, but what about a "British Boys" rifle would make it illegal?
I'm not contradicting you, I just don't know. I have never heard of it. It just looks like a large, high powerd rifle to me.
The Boys Anti-tank rifle was a .55 cal bolt action rifle designed between the World Wars, so it was obsolete as an anti-armor weapon by the time WWII started. It's probably a destructive device.
Boys anti-tank rifle - Wikipedia, the free encyclopedia