Found a modelling scalpel with a rigid plastic cover over the blade at the bottom of my pannier the other day, I use it to cut the long ends off zip ties. So, this is presumably highly illegal and I should get myself a small penknife?
No. The modelling knife is carried by you for a specific purpose, ie to cut zip ties. Provided you explain that and it's obvious to the officer that you have zip ties holding your bike together, and spares that you are likely to attach at some point in the future, you have your good reason. A small folding pocket knife would be better, and wouldn't need explaining.
I carry a small liner lock knife when I'm at work. It's sharp enough to shave with, and has cut several ligatures and a few seatbelts in its time, so my work justifies it. I personally refuse to carry a folding pocket knife as a matter of course, simply because I find them incredibly dangerous to the user. My left forefinger and the ball of my right thumb bear the scars of stupid weak folding blades.
I personally disagree with the case law that made a lock knife fall outside of the definition of a folder. I don't believe the legislation was originally aimed at them, but someone, somewhere had a point to prove in taking someone to court and arguing that releasing the lock before folding the blade made it more dangerous.
Blade length is mostly irrelevant. A two inch blade struck with sufficient force will penetrate deep enough to hit many vital organs, give the way that the body will compress under the force of the striking fist. I've seen autopsy photos of punctured hearts and livers using a small vegetable peeler.
Of more concern than the locking function should be the opening method. All of my lock knives ( and I have a few for different purposes, none bigger than 7.62cm) are designed for one-handed opening, either with an exaggerated cutout or a thumb stud. Practically speaking I can hold a fish, rabbit, struggling suicidal prisoner or whatever in one hand, and open my knife with the other.
However, I've also faced three people with knives, all of which wanted to hurt me. One was a half pair of tailor's shears sharpened to stiletto dimensions. You can't conceal eighteen inches of gleaming steel, so he got pegged before he could even raise it halfway. Another made great show of opening his folder in front of me, giving me time to "generate a bit of distance" and shout for help, and the third waited til I was up close and personal before whipping out a thumb-opener he'd got in his pocket and cured my constipation by remodelling my jacket for me. My preferred options are to generate distance and shout for help, or resort to pegging folk only if strictly necessary. In that instance I decided I could do both, and shouted for help whilst pegging him reasonably enthusiastically.