Wasnt there something about members of Gun Trade association have a Gentlemans agreement to not test the water on this too?

Just to pick up on something GaryC said, the .22 rimfire cartridge is the answer to this!!! Grab yourself an FAC, get a slot for a .22l/r rifle and then take your pick!! Many different varietys are available to the UK semi auto shooter:

Ruger 10/22
Anchutz 525
Remington 597
Many different AR-15 type variants
Walther GSP (for gays)
Walther G22 (for bullpup fans)

This list goes on and on..... rimfire is anywhere between £1.50 and £4 for 50 rounds.
Not quite as cheap as air rifle pellets but after you factor in the cost of an air bottle, service of air bottle, filling equipment like hoses etc, re filling etc it does balance out.