Because a law is a law. I didn't realise that was so hard to understand.
I copied this from the 'British Knife Collector Guild' website.
THE LAW
YOUR KNIFE AND YOU
Despite what you may have been led to believe, our knife laws are amongst the most sensible in the modern World.
In Great Britain we benefit from laws which promote the sensible use and collecting of fine cutlery, yet discourage those who seek to abuse what are, in effect, artistic, well engineered tools. In short, be aware of the following points, although please read the linked articles for more in-depth information ...
What You Can't Have ...
The following items are banned from sale within the UK (although if you already own one you may keep it, but not use it outside of your own property) ... Switchblades, automatics or 'flick-knives', gravity knives, balisongs or 'butterfly knives', push daggers, belt buckle knives, sword canes, disguised knives, or knuckle-duster knives.
Late on in 2004, an amendment to the law was introduced which restricts the sale of any knife which is not readily detectable by the normal methods of detection, ie: either x-ray or metal detection, unless it can be proven that the knife's sole purpose is for the preparation of food. So for instance, the Cold Steel CAT Tanto or Lansky Knife are now prohibited within the UK. These knives are correctly referred to as Airport Knives, but in English law are commonly referred to as Stealth Knives.
In 2006, so-called Disguised Knives were prohibited. You may not buy any knife designed to look like something else, for instance a knife which appears to be a pen, (and it doesn't matter whether the pen works or not, it's still prohibited here).
What You Can Carry ...
The Criminal Justice Act (1988) says that you may carry a knife with a blade length of 3.0" or less so long as it is capable of folding. That means no fixed blade knives. But use your loaf - a knife has no place at a football match, in a pub, nightclub or school and becomes an offensive weapon in these circumstances in just the same way as a screwdriver, or any other innanimate tool.
But I NEED a Bigger Knife ...
If you wish to carry a larger knife then you must have 'reasonable cause'. That means that you must be able to prove that you had a genuine reason for carrying the knife.
You may carry a larger cutting tool if it is associated with your work (for instance a chef may carry a 9.0" butchers knife roll to and from work), or if it is associated with your sport, (for instance a fisherman may carry a 6.0" fillet knife, or a hunter may carry a 4.0" fixed blade hunting knife).
Don't forget it's there though. If you stop off in Tesco's for a can of beans on your way home take the knife off of your belt and lock it in your glove box, or your local Bobby will be unimpressed at your excuses. When transporting a knife by car keep it locked away in the glove box or securely stored in the boot of the vehicle. Do not slip it into the door side-pocket, under your seat or in a centre console, this is a dangerous practice, and if stopped by the Police this gives the impression of keeping the knife close to hand.
