The problem with knife laws is that every time the Government has a problem with the figures going through the roof, then they do the cheapest thing to give the impression of tackling it. That is, they pass a new law. There is more legislation before Parliament now. Increasing resources to catch offenders is expensive, so you don't see too much of that! But if you carry a knife in public, undoubtedly, you run the risk of falling foul of one of these laws.
Basically, without getting too bogged down in specific legislation, if you stick to these rules, you SHOULD be okay.
1. Don't, as a rule, carry a knife with you. Even if you stick to the rules of the blade being less than 3" and it isn't locking. Because any article (includes knives) can be interpreted as being an offensive weapon given the evidence surrounding the incident. If they can prove it is made, intended or adapted for use as a weapon. For example, (as above), if you've got any sort of knife in a nightclub or bar, you must have the intention to use it as an offensive weapon and it'll be hard to argue otherwise. The same goes if you go to court with a blade. Why would you want one there? You can try the "whoops it's a mistake" defence if you like, but don't hold your breath. It can be hard to prove intent, but having a knife in such places makes it much easier.
2. If you simply HAVE to have a knife, say for your job, you have reasonable grounds for carrying a knife, then that will be a defence and should see you not getting charged. Whatever sort of knife it is, so long as it's suitable for the job. It doesn't have to be for your job, but it's got to be reasonable. If you are off stalking, for example, then you need a knife. Or a chef going to work. But if that stalker or chef still has that knife on him and he goes for a few pints afterwards, or goes to court for his speeding charge, then beware!
Knife crime is in the news. To be fair, it's pretty often in the news. Which means the authorities will be wanting ACTION! Which means the cops will be under pressure to show results. So if you find yourself being stopped with a knife on you and they think you have committed some offence, they WILL act on it.