Legal personal carry in the UK is a folding bladed knife with a less then 3 inch blade. Something like a Swiss Army knife of other pocket knife. Anything with a fixed or locking blade and/or over three inches in length is illegal to carry in public unless you can reasonably argue why you have it. For example, if you are a chef and have a set of professional knives in the boot of your car.
It's at the discretion of the arresting officer as to whether your explanation is valid and the discretion of the court as to what will be done with the charge. Personally, I always found a pocket knife to be not enough for a proper camping trip - I always wanted to carry my lock knife or belt knife but never did because I wouldn't risk it with the police. But I'd take my lock knife on excavations because it was a really useful tool and as such I could reasonably argue that it was needed for my job. But, if I left site and went to the shops, I'd ditch the knife.