For the UK, knives where the blade folds into the handle, like a Swiss Army Knife, aren't illegal as long as the blade is shorter than three inches (7.62 cms). It is however, possible for it to be considered an offensive weapon depending on the circumstances. For instance, there's no reason to have one on you when going out on the town, clubbing, pubbing etc.
For once UK law is quite sensible, recognising that men often carry a penknife or multi-tool in connection with a hobby or occupation, and old ladies often have a little penknife in their handbags for peeling fruit, cutting a nice slice of cake and so on.