Any knife carried must be 3.5 inches max blade length, and must not be a fixed blade knife, nor assisted opening (switchblade, butterfly/balisong, and spring assisted lock knives are all illegal). lock knives are currently legal, but this could change in the current knee jerk response to knife crime.
There is also the question of intent/purpose for carrying any bladed tool. If you have reason to be stopped by the police in possession of a knife, and you have no reason for carrying it other than your wish to do so, it could be seen as an offensive/concealed weapon, and at the very least be confiscated.
When it comes down to it, if you carry a knife for self defense or to cause bodily harm, then even if it is a small pen-knife, if you use it, you are going to court.