When you outlaw machetes, only outlaws will have machetes.
Britain already has knife control.
Really.
“The law already prohibits the possession of offensive weapons in a public place, and the possession of knives in public without good reason or lawful authority, with the exception of a folding pocket knife with a blade not exceeding three inches.”
http://news.bbc.co.uk/2/hi/health/4581871.stm