I have some communication with Parcelforce which indicates that they will not carry pressure vessels whether empty or not, i.e. they will not carry PCP's. this is an ongoing dialog but certainly shows a reluctance.
An earlier post seems to indicate that the situation about insurance is unclear, The lines in their T&C's saying that they will carry airguns subject to two conditions, a) must be posted at a Post Office and b) they are subject to a maximum compensation of £100. make it seem pretty clear to me.