Where do upstanding brits get a gun?
'Upstanding' Brits don't get the sort of shooters that gang wankers use. There is of course, a black market for un-licenced guns. You can meet a guy in a car park and exchange cash for an Eastern European shooter. Probably a Russian or Bulgarian hand gun.
If the feds catch you it's an automatic 6 year prison term (since the Dunblane shooting).