Please note that Scotland and Northern Ireland have their own legislative powers and consequently not all UK legislation applies in these countries.
The CJA 1988 mainly relates to carrying knives in public places, Section 139 being the most important.
"It is an offence for any person, without lawful authority or good reason, to have with him in a public place, any article which has a blade or is sharply pointed except for a folding pocket-knife which has a cutting edge to its blade not exceeding 3 inches." [CJA 1988 section 139(1)]
The phrase "good reason" is intended to allow for "common sense" possession of knives, so that it is legal to carry a knife if there is a bona fide reason to do so. Examples of bona fide reasons which have been accepted include: a knife required for ones trade (e.g. a chefs knife), as part of a national costume (e.g. a sgian dubh), or for religious reasons (e.g. a Sikh Kirpan).
In this case, public place is meant as anywhere accessible to the public, so for example a private campsite, which members of the public must book to use, is a public place. Also, knives should only be carried to and from and used at the location where they are needed. For example, leaving a knife in a car for use when you go fishing would be illegal. It should be taken back into the house each time you use the car (other than to go fishing).
The special exception which exists in the Criminal Justice Act 1988 (s139) for folding knives (pocket knives) is another "common sense" measure accepting that some small knives are carried for general utility however even a folding pocket knife of less than 3" (76mm) may still be considered an offensive weapon if carried or used for that purpose. It was a long held common belief that a folding knife must be non-locking for this provision to apply.
A Crown Court case (Harris v DPP), ruled (case law). A lock knife for all legal purposes, is the same as a fixed blade knife. A folding pocket knife must be readily foldable at all times. If it has a mechanism that prevents folding, it's a lock knife (or for legal purposes, a fixed blade) The Court of Appeal (REGINA - v - DESMOND GARCIA
DEEGAN 1998) upheld the Harris ruling stating that "folding was held to mean non-locking". No leave to appeal was granted.
British law also covers age restriction on the sale of knives in the Criminal Justice Act 1998:
"It is an offence for any person to sell to a person under the age of 18 any knife, knife blade, razor blade, axe or any other article which has a blade or is sharply pointed and which is made or adapted for causing injury to the person." [CJA 1988 section 141A]
British courts have in the past taken the marketing of a particular brand of knife into account when considering whether an otherwise legal folding knife was carried as an offensive weapon. A knife which is marketed as
"tactical", "military", "special ops", etc could therefore carry an extra liability. The Knives Act 1997 now restricts the marketing of knives as offensive weapons and thus it is much more unlikely that such marketing could be used as evidence against a defendant.

In practice, this law makes it highly unlikely that most shops would sell a knife to someone younger than 18.
In the UK, the main knife legislation is found in the Criminal Justice Act (CJA) 1988 however certain types of knife are banned under the Restriction of Offensive Weapons Act (ROWA) 1959, the relevant section of the latter being
Section 1.
"It is an offence for a person to manufacture, sell, hire or offer for sale or hire or expose or have in his possession for the purpose of sale or hire, or lend or give to any person:
A) any knife which has a blade which opens automatically by hand pressure applied to a button, spring or other device in or attached to the handle of the knife, sometimes know as a "flick knife" or "flick gun"; or
B) any knife which has a blade which is released from the handle or sheath thereof by the force of gravity or the application of centrifugal force and which, when released, is locked in place by means of a button, spring, lever, or other device, sometimes known as a "gravity knife". "
[ROWA 1959 S 1(1)]
Section 1(2) also makes it illegal to import knives of this type, as a result it is (almost) impossible to obtain possession of such a knife without either committing or abetting an offence. Note that the above legislation does not refer to possession of such knives other than possession for the purpose of sale or hire, it is therefore not illegal per se to merely possess such a knife.
This law is aimed primarily at knives designed with features specific to fighting/assault rather than use as a tool.
Although English law insists that it is the responsibility of the prosecution to provide evidence proving a crime has been committed an individual must provide evidence to prove that they had a bona fide reason for carrying a knife
(if this is the case). Whilst this may appear to be a reversal of the usual burden of proof, technically the prosecution has already proven the case (prima facie) by establishing that a knife was being carried in a public place.