A careful, considered short research article in 'The Conversation', a daily mainly UK academic newsletter:https://theconversation.com/how-illegal-firearms-find-their-way-onto-british-streets-despite-tough-laws-61239?

The author has had help from UK policing (via NABIS) and the ingenuity of those who seek lethal firearms is well explained. She ends with:
Without a doubt, so long as there remains a demand for firearms in Britain, the criminal underworld will use all its ingenuity to meet that demand and continued research into their methods and the vigilance of police forces will be required to keep them out.