Its an odd fact that if you make laws that create new offences : you get an increase in 'crime'. Its more sins of omission : when certain categories of firearm became illegal to own, many did nothing and then got caught in possession. so, a large part of the increase in offences was not 'criminal' per se, more a failure to conform to the new conditions. They did run amnesties and such but some folk just ignore stuff they dont like much or expect : When a series of roads in North London were changed to a new one-way system, people kept driving up and down them as they had always done for weeks afterwards, from sheer habit. Guess what? yes, a massive increase in traffic offences detected occurred!
Besides, the main thrust of the changes in gun laws in the UK was in response to a serious increase in criminal fire-arms use by street gangs, so the poster has the chronology back to front somewhat.
Where that increase in criminal fire-arm use was and still is occurring is amongst those youth street gangs, and its largely due to that culture which recognises gun ownership and display as a status issue, the use of firearms in 'revenge' attacks which prove 'manhood' and provide 'respect' which then provide ranking within the gang. Thus, its no surprise that the majority of their victims are gang members themselves. And yep, no prizes for guessing where the inspiration for these gangs and their philosophies originates.