Go into his mums kitchen drawer, nick her 7" carving knife, shove it down his trousers and be on his way?
Siberianfury, I think you may have inadvertently hit upon the solution to the problem. If these angry young men keep shoving 7" carving knives down their trousers, in a very short time, the U.K. population of angry young men will begin to plummet as they kill each other off and since they will be unable to reproduce (see above) the situation will take care of itself in one generation.
My only suggestion for a new law would be one that would require all mums to have no knife shorter than 7" and all kitchen knives must be kept carefully honed and stropped at all times!