Anything that has a locking blade, regardless if it’s under 3”, is illegal to carry without good reason
If it has a friction blade, like a Swiss Army knife for instance, then it’s legal to carry as long as you have a good reason. I’d say that having it to carry out maintenance or repairs on your bike counts as a good reason.
edited after reading Bob’s link.
This. So I can legally carry something like this
but can’t carry a 2cm blade like this because it has a locking mechanism.
Locking mechs make knifes far safer to use. My Alien tool for my bike and my Gerber multi tool are both illegal to carry due to the locking blades on both. The current legal blade length is probably 2-3cm too big and the illegality of locking mechs is just a nonsense. Take a look at https://heinnie.com and tick the uk legal carry box. I think you’ll be surprised at some of the stuff it’s perfectly legal to carry around in your pocket,
Them’s the rules.
Not illegal, just not without a good reason.
What reason do you have for wanting one?
Didn’t say it made sense. It’s the reason I haven’t bothered replacing my lost Leatherman, because they have locking blades.
The problem with multitools is that you frequently don’t know the reason until you need them 
I think if you had one, on a bike trail in the middle of nowhere you would probably be ok. In a city centre nightclub, less so.
Actually, I’ve just read Bob’s link, and locking blades are also legal if you have a good reason. Not sure if I could think of a good reason if I was walking through the town centre with one and got challenged though.
Is “a good reason” actually defined in any meaningful way do you know?
I suppose circumstances change everything.
Your average builder or tradesmen could technically be ‘going equiped’ but it is very unlikely to be a problem unless it is midnight, he is wearing dark clothing with gloves and a balaclava. and doesn’t have a good reason.
Common sense does apply quite often!
A tiny 2 or 3cm keychain knife is a handy thing to have on your person, opening packaging, sharpening a pencil, etc. In line with a multitool, you never know until you need it. The fact I can’t have one with a locking sheepsfoot blade and risking a nasty cut (been there) without breaking the law is farcical when its perfectly ok for me to carry this without any excuse needed.
An axe is fine though👌
You can have one though. If you’re reason is to sharpen your pencil, that’s reasonable as long as you have said pencil on you👌
“I am off to a meeting officer where I intend to sharpen a pencil”, might work 
I rather suspect that plod won’t be pulling over any aging abatoirists to check the function of their knives.
Last time I went to London I must have walked past a dozen coppers and not one of them was interested in a pat down.
Some of us are more attractive 
Lol, I’ve seen your FB page,
i need to do that regularly when out and about.
