Here's the official UK view: http://www.direct.gov.uk/en/CrimeJusticeAndTh...
Basic rules on knives
If you’re planning to buy or carry a knife, it’s important to know the rules. Anyone found breaking these laws can face up to four years in prison.
* it is illegal for any shop to sell a knife of any kind (including cutlery and kitchen knives) to anyone under the age of 18
* it is a crime to carry a knife in public without good reason – for example, if you work as a chef
* the maximum penalty for an adult carrying a knife is four years in prison and a fine of £5000
* it is illegal to carry, buy or sell any type of knife banned by the government (the list of banned knives is below)
* knives with folding blades, like Swiss Army knives, are not illegal as long as the blade is three inches long (7.62 cm) or less
* if any knife is used in a threatening way (even a legal knife, such as a Swiss Army knife), it is regarded as an 'offensive weapon' by the law
* any sharp instrument – even a screwdriver – can be viewed by the police as an illegal offensive weapon if you do not have a good reason for carrying it
There is a complete ban on the sale of some knives, which are considered to be offensive weapons.
* flick knives - knives where the blade is hidden inside the handle and shoots out when a button is pressed; these are also called 'switchblades' or 'automatic knives'
* butterfly knives - where the blade is hidden inside a handle that splits in two around it, like wings; the handles swing around the blade to open or close it
* disguised knives - where the blade is hidden inside something like a belt buckle or fake mobile phone
* gravity knives
* samurai swords
* belt-buckle knives
* push daggers
* kubotan (cylindrical container, holding spikes)
* shuriken (also known as 'death stars' or 'throwing stars')
* kusari-gama (sickle attached to a rope, cord or wire)
* kyoketsu-shoge (hook-knife attached to a rope, cord or wire)
* kusari (weight attached to a rope, cord or wire)
This is not a complete list.