I'm not a 100% sure on it but what I've heard is they are very hard to get a hold of.
Reason being is that you can't hunt in the UK. So if you are caught on public land with them, your screwed. Police will charge you with being in possession of an offensive weapon. Basically you wouldn't have a leg to stand on in court.
Your probably permitted to own them here, just not use them unless they are being used on private land at a distance that is of no harm to the public or animals.
That sounds crazy to us 'muricans.
although, sometimes I think it might be nice if I didn't have to clear the woods of hunters before I let my kids play outside.