No - the current UK legal definition of a firearm is a "lethal barrelled weapon" capable of firing a projectile.

Airguns are already classed as firearms, under this definition, by law. Different classes (or sections) of firearms are treated differently for access, certification, storage etc.
There is no difference in law between an offence (eg armed trespass, threats to kill) committed with an airgun or any other firearm.

The problem is that there is no formal definition of "lethal". Under EC rules it is accepted that anything incapable of producing more than 1 joule of muzzle energy is a toy. We are suggesting this should be adopted in the UK. this will exclude airsoft, most BB guns, spud guns and the like from being caught up in firearms legislation. There is no question of this becoming a maximum power limit for airguns.