The legal definition of a paintball gun has never been put into a bill nor has it been set by a court.
Air weapons are defined and technically paintball guns come under the legal definition of a air gun.
The Firearms Acts 1968-97
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".
Paintball guns strictly come under the definition of the licensed air guns portion, having too much impact force to be unlicensed. The trivial injury portion and the soft shell of a paintball is what is allowing paintball guns through the gap, however the public don't have masks so firing at someone or pointing a paintball gun at someone without a mask could lead to none trivial damage and therefor back to a firearm again. Reballs being solid technically remove this gap and change the classification to a full firearm.
Paintball gun dressed up to look like real guns, i.e. the mp5 and m16 dressed gun now come under different regulations. Anti-Social Behaviour Act and Violent Crime Reduction Bill doesn't matter that they are paintball guns just what they look like, don't use one of those anywhere but a site.
Anyone over 18 can use a air gun, 14 to 18 may use one on private property without supervision, under 14 requires direct supervision of someone over 21.
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.
In my opinion you should play it safe and treat a paintball gun as an air gun which is what they are currently being loosely classified as by the police.