If you are going to fire it, it has to be proofed. It might be only you using it but that doesn't stop the guy standing next to you from getting a face full of barrel if it blows up.
In the past a lot of the kits were sold as wall hangers but they could be converted to fire relatively easily. They are subject to much more lenient laws in most of Europe but I doubt whether they would be legal in this country.
Interesting thread though, I'm sure someone more conversant with current legislation will enlighten us.