There are two separate pieces of legislation which are relevant. One refers to 'offensive weapons', whereas the other refers to 'bladed articles'.
The definition of an offensive weapon is for a court to decide, considering all of the relevant circumstances. (In one case it was ruled that a frozen chicken constituted an offensive weapon!). So a simple drawing pin could be classed as an offensive weapon, if it could be shown that you intended to leave it where someone could sit upon it.
However the legislation relating to bladed articles includes all articles which have a blade or are sharply pointed. The only exception is that folding pocket knives, where the cutting edge of the blade does not exceed three inches, are excluded from that specific piece of legislation.:
http://www.legislatio...and-offensive-weapons
Chris