Laws are very strict. Shotgun license (which I have) allows semi autos with max 2+1 shells. Any other semi auto requires a special firearms license which is an absolute nightmare to get (police visits, interviews, references, doctor's report and a good reason to own one)
Needs to be a permanent alteration, that is, not reversible
In the UK the tubes are made to UK spec. Any idea where I might get a UK spec tube i the USA?