It's strange to me that some here don't think if people were legally entitled to carry any knife they liked in public, there wouldn't be an increase in knife crime.
I think I understand your point. We have an existing knife law that says anyone can carry a knife at any time; as long as it is a 3 inch non-locking folder. You can also own and use a huge variety of other knives as long as you have good reason to. For most people this is a good law as for most people that folding knife will cope with any unexpected knife tasks that you face on a daily basis. For other times when you need a better knife you should be covered by the 'good reason' law. I bet 99.9% of the public would be fine with just a SAK on a daily basis.
Most 'criminal class' types aren't going to carry a SAK or similar though as the main purpose of a knife to them is to intimidate, show off or maybe even use to stab people. Rapid deployment is key to most of the stuff they want to use it for. So if the police search them they will most likely find an illegal weapon and will have the power to arrest them and confiscate the knife. Changing the knife law isn't going to effect the people already carrying illegal knives.
If you make it legal to carry any knife though then the police lose the power to arrest and borderline cases will be able to walk around with any number of weapons. Conversely your average person will still be carrying a SAK because why would you carry a fixed blade or whatever for occasional light use? (Yes it would be nice to carry a locking folder but we've been through the lawyer bashing already). So now you have more people on the street with fixed blades, more people in the pub with large knives, more kids who carry a big knife to feel 'ard. It would be hard to see how knife crime wouldn't rise in that situation. The fights and assaults are probably already occurring but you'd be replacing fists and pool cues etc with pointy stuff.