Even if a criminal can still acquire an illegal firearm, that wouldn't be in his best interests if it's prosecuted with sufficiently more vigour than the other crimes they intend to commit. For example, your average British mugger or burglar is exceedingly unlikely to be so armed - the UK police will dedicate a lot more resources to catching a thief if his victims report he has a gun, and once caught he'll be locked up for a lot longer with it added to his rap sheet.