"Above 1.3J and capable of full auto an airsoft weapon becomes a Section 5 firearm."
The law has recently (Since the VCRA) become a bit of an ass in this respect. I have a some air rifles, and a couple of air pistols. The latter are modelled on real world pistols (one S&W 686, and one Beretta 92), try and buy an airsoft version that's not a funny colour though, and it's a different kettle of fish, and you need UKARA membership. I find it odd that lower powered plinkers have more restrictions.
I'd also love to know if the VCRA has done the 'R' bit, I suspect not, and would think criminals would now be pointing air pistols at people, instead of airsofts, but hey.