I don't have the chapter and verse to back this up but I was told by a friend who was in to such things that during the Thatcher era, a then just-retired high ranking officer in the Met was commissioned to prepare a report on the links between legal weapons and crime. His conclusion was that of seized weapons used in crime, less than 1% had ever been legal. This was not what the establishment wanted the report to say so the results were sat on.
No doubt anyone with more interest in weapons than I can verify this. I understand the report was eventually published.
I do remember that just after the Dunblane tragedy a Glasgow reporter told of the weaponry for sale in a pub - including a double barrelled automatic shotgun! Does such an animal exist?
As others have said, I do not believe that the increasing use of guns by the criminal fraternity in the UK has anything to do with our gun laws. If the fairly stringent controls prevent someone like the killer of the Amish girls in the US, or Peter Hamilton at Dunblane from running amok - then let's have them the way they are!

Eddie