Knife laws in the UK aren't really that strict; they're very flexible, but that leads to a lot of confusion.
The main laws regarding knife carrying are:
- Prevention of Crime Act 1953 (?)
This basically says that anything carried with the intention of harming another is illegal. This can cover knives, hammers, screwdrivers or even a newspaper.
The problem with this law is there is a presumption of guilt and it's up to you to convince the magistrate that you had a perfectly good reason for carrying whatever you where. For example; walking to a friends house with a screwdriver to fix a light fitting, perfectly legal (as long as you can prove that's what you're doing; phone call to said friend should be fine). Walking down the street with a screwdriver asking people for a fight, very illegal.
- Criminal Justice Act 1988
This covers the carrying of bladed articles in public; while it doesn't apply to a non-locking folding knife with a blade under 3", those can still fall under the Prevention of Crime Act if used threateningly (or it is believed you will use it threateningly).
This law basically says that to carry a bladed article in public you need a good reason, you can't carry it "just in case". Again, it's up to you to convince the powers that be that you do have a good reason. If you're hiking, camping, at work doing a job that needs a knife etc. then you can carry whatever you need while you do that. Example; A groundskeeper with a fixed blade strapped to his leg wile on duty; perfectly legal. A groundskeeper with a fixed blade strapped to his leg while in the pub on his day off, illegal.
This also lists some knives that are illegal to buy/sell (autos, balisongs, "stealth" & and disguised knives, push daggers and knives incorporating a "knuckle duster").
For EDC we stick to sub 3" non-locking folders because they don't come under the CJA1988, but if we ever need a bigger/locking/fixed knife then we can carry that also; we just need to know what we're going to use it for beforehand.
PS. While I'd prefer to be able to carry a slightly bigger locking folder every day, a 3" slipjoint is fine for most day-to-day tasks (after all, many people around the world get on just fine with a SAK).