Yeah, but, there are several different factors to it..
If the blade is closed, or a swiss army knife, you can argue that the knives are sold all around the place, in places like Tesco, Woolworths, Argos, etc etc..You dont see those huge things that Peter posted in your local shop do you..haha
Secondly, if the knife is an open/close knife, you can argue that it can't cause harm, because it is actually closed..but on the other hand, you can just open them...lol
Again, if you can prove the knife is for a good and meaningful cause, they just let you go on, or tell you to put it away, and hide it. But, if you are like a scally, with your hood up, and tell the police that the knife was just to help your gran with her gardening or something, that won't work. I carry a knife to work, because, we have to open tonnes of boxes, and tonnes of crates of bottles, and a knife does the job right..
But as long as you dont go waving it about the streets, and public places, you'l be ok..