The major problem is that the prosecution is going to contend that a fixed blade knife is not nessesary. Therefore your possession is not lawful. You have the problem of demonstrating that it is. We are all perfectly aware that a fixed blade knife or a lock knife is a tool, not a weapon. Most of us take deep and abiding offence at being tarred with the same brush as the yobbo element. We are in the buisness of saving lives, not taking them. A subtle nuance that is probably lost on most magistrates and police officers.