I just looked at a (google) map of the UK. There is a LOT of shoreline, especially Northwest of Glasgow, that I suspect is quite difficult to patrol effectively, fairly reachable from Ireland and Norway, and borders on what looks like very sparsely populated land. There are similar difficulties for other governments in parts of the Eastern U. S. and Canada, as well as Alaska and British Columbia, and a long U. S. border with Mexico that has been shown to be quite permeable.
I do not think it is obvious that the UK or other governments would be able to exercise effective control of a large, but popular illegal weapons import program. The effective control is nearly universally accepted legitimacy of the governments.