The following is lifted from another website - I offer no guarantee of the accuracy or validity at any point of time of the legal information contained. It is simply a GUIDE to UK law relating to airguns in general.

Airgun UK legal issues.

Airguns in the UK are subject to the firearms acts, under the Firearms (Dangerous air weapons) rules 1969 they are classified as low powered Air Weapons and as such they are restricted to a maximum power of 12 foot pounds force for a rifle and 6 foot pounds force for a pistol. Above 12ftlb a rifle is classified as a Section 1 Firearm and requires a licence called a firearms certificate, and a pistol above 6ftlb is again a Section 1 Firearm requiring a firearms certificate in the UK.
The definition of a firearm under the act is "a lethal barrelled weapon of any description from which any shot, bullet or other missile can be discharged" it further defines "lethal weapon" as "a weapon capable of firing a projectile with sufficient force to inflict more than a trivial injury i.e.. with sufficient force to puncture skin".
The Home Office consider the lowest level of muzzle energy capable of inflicting a penetrating wound is one foot pound (1.35 joules) hence guns producing less than 1ft/lb are not covered by the act and therefore not classified as air weapons or subject to any restrictions.
The 1997 Firearms Amendment Act made handguns illegal in the UK but Air Weapons were excluded under Section 5 (1) subsection (aba) "a prohibited weapon includes a firearm with a barrel length less than 30cms and an overall length less than 60cms, other than an air weapon, a muzzle loading gun, or a firearm designed as signalling apparatus".

UK Legal Limit
To calculate the power of an airgun you need to use a chronograph to measure the speed of the pellet (in feet per second) when fired, and you need to know the weight of the pellet in grains. Once you have that information you perform the following calculation:- speed(ft/sec) X speed(ft/sec) X weight(grains) 450240
This gives you the result in foot pounds force (ftlb).
As mentioned the legal maximum for an unlicensed air rifle is 12 ftlb which from changing round the above formula, gives the approximate values as follows:-
A .22 pellet weighing 14.4 grains (.933 gms), maximum permissible speed is 612 ft/sec
A .177 pellet weighing 7.9 grains (.512 gms) , maximum permissible speed is 826 ft/sec
The corresponding figures for a pistol are 433 ft/sec for a .22 and 584 ft/sec for a .177
The pellet weights used in the above calculation are typical weights for the sizes of pellet but you must always check the actual weight of your pellet before performing your own calculation.

Purchasing Guns and Ammunition
The laws concerning the purchase of guns and ammunition are as follows:-
Any person under the age of 14 years of age may not purchase or own guns or ammunition.
Persons aged between 14 and 17 years may not purchase guns or ammunition but may borrow or have them purchased on their behalf by some one over 17 years of age.
Persons over the age of 17 years may purchase guns and ammunition, the guns having a power less than the UK legal limit. Guns over that limit require a fire arms certificate to be granted before they can be owned.

Airguns and Young Persons
The laws concerning the use of airguns by young persons are as follows:-
Any one under the age of 14 can only use an airgun whilst under the direct supervision and control of someone over 21, whilst on private property with the permission of the owner of the premises.
A person aged between 14 and 17 can use an airgun on private property where they have a right to be, without supervision.
They may carry an unloaded rifle (but not a pistol) in a public place only if it is in a securely fastened gun cover so that it cannot be fired, an under 14 year old must again be directly supervised by an over 21 year old.
A pistol can only be carried by someone over 17, again it must be unloaded and in a securely fastened case.

Public Places and Highways
You may never have an airgun in a public place without proper reason for doing so. If you are travelling to and from a place where you have the right to shoot, the gun must be in a case as above. A gun is loaded if there is a pellet or any form of projectile in it, including an "air weapon which has a loaded magazine, is loaded even though there is no round in the breach".
It is an offence to fire an airgun within 50 feet of the centre of a public highway, If by doing so you cause any member of the public using that highway to be injured, interrupted or endangered. This applies even if you are on private property adjacent the highway. Public highways include roads, bridleways and public footpaths.
Trespass
If you go on to any land, including over water and in buildings, without permission you are Trespassing. If you have an Airgun with you it is classed as Armed Trespass which is a serious Criminal offence even if the gun is not loaded
Airgun Prey
The following pests are considered suitable for controlling using a sub-12 ftlb Airgun.
Brown Rat, Grey Squirrel, Rabbit, Crow, Rook, Magpie, Jay, Wood pigeon, Collared Dove, Feral Pigeon, House mouse, Sparrow, Starling, and recently Mink have been added to the list.
These are not always considered pests and only Authorised Persons can shoot them in all circumstances.
Wildlife and Countryside Act 1981
All bird and animals are protected by law. Certain species are classified as pests or vermin and only these species can be legitimately shot and then only by authorised persons. An Authorised Person is someone who has the proper permission from the land owner to control pests on that land.
Within that list of designated pests some are not suitable prey for an airgun producing less than 12 ftlbs.

Offence Penalties
Statute Law
The Penalties for breaking current UK firearms laws with Airguns are as follows:-
Carrying a loaded Air-weapon in a public place 6 months imprisonment and / or £5,000 fine.
Trespassing with an air weapon 3 months imprisonment and / or £2,500 fine.
Trespassing on private land with an air weapon 3 months imprisonment and / or £2,500 fine.
Possessing or using an air weapon if sentenced to 3 months or more in custody 3 months imprisonment and / or £2,500 fine.
In addition if original sentence up to 3 years 5 year ban on use of firearms.
Or if for 3 years or more Life ban on use of firearms.
Killing or injuring any bird or protected animal unless authorised £5,000 fine.
Firing an air weapon within 15m / 50ft of a public highway £1,000 fine.
Selling or hiring air weapon or ammunition to person under 17 6 months imprisonment and / or £5,000 fine.
Making a gift of air weapon or ammunition to person under 14 £1,000 fine.
Having air weapon or ammunition with intent to damage property 10 years imprisonment.
Having air weapon with intent to endanger life Life imprisonment and / or appropriate fine.
Using air weapon to resist or prevent arrest Life imprisonment and / or appropriate fine.
Threatening others with an air weapon (even if unloaded) to cause them to fear unlawful violence 10 years imprisonment and / or appropriate fine.
Not forgetting the chance of being shot and killed by the police should you not obey instructions when challenged by them, they cannot tell if you have just an airgun or a more lethal firearm so will treat all arms as lethal and respond accordingly.

Civil Law

It should be born in mind by every airgun shooter that the unexpected could happen and they could find themselves facing a civil action for damage to property or even injury to persons or livestock. Every airgun shooter should have Third Party Public Liability Insurance before venturing out of doors, and joining one of the bodies representing shooters interests is the best way to achieve this.