This is all to do with the Violent Crime Reduction Act 2006, in some ways owning a realistic Airsoft gun is more onerous then a Firearm. There are also some absurd anomalies because of the VCRA 2006.
Own this and you need to be UKARA registered:

Own this and you just need to be over 18:

The former is a 6mm bb pistol the latter a .177 air pistol. The air pistol is more powerful then the airsoft pistol yet anybody over 18 can go to a gun shop and buy it. The former requires UKARA membership with regular attendance of a skirmish site or it must be two tone (blue/orange).