I do not agree with High Power's statement...

The act says the following:
airgun means any device manufactured to discharge a bullet or any other projectile-
(a) of a calibre of less than 5,6mm (.22 calibre); or {my emphasis}
(b) at a muzzle energy of less than 8 joules (6 ft-lbs),
by means of compressed gas and not by means of burning propellant

So lets break the 'easy bits' down first...

"by means of compressed gas and not by means of burning propellant" ie NOT a center fire or rim fire rile... we agree on this. Weather by spring, or PCP, it is air powered. The spring drives a plunger, which compresses air behind a pellet...

The confusing bit is the middle section...
(a) of a calibre of less than 5,6mm (.22 calibre); or {my emphasis}
(b) at a muzzle energy of less than 8 joules (6 ft-lbs),

According to this, in section (a) it states that a rifle less than 5.5mm is NOT a firearm without any restriction on the power of the rifle...
then, in section (b) it states, as a separate condition, if your rifle produces less than 8joules of energy, it is NOT a firearm, without any restriction on the caliber...

The key here is the OR. It makes the 2 items independent of each other. if it was an AND, it would be a different ball game...
So as stated above...
(a) less than 5.6mm, any power, you are safe, no license required...
(b) any caliber, less than 8 joules energy, you are safe, no license required...

however... you have a 50 cal Air Gun, shooting a pellet at 1000fps.... THIS is considered a firearm, requiring a license...