I have copied and pasted the latest Royal Mail information, which includes the fact that you can send Air Pistols by first class post as the minimum requirement, which is something I omitted from my previous post, but compensation using this service is extremely limited although you can get a signature on delivery, for what it's worth.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------

Guns for sporting use

•International - Allowed in the mail, see restrictions and packaging guidelines below:
◦Guns intended for sporting purposes - including Section 1 and Section 2 firearms, low-powered air guns and their component parts - may be sent in compliance with UK law and subject to applicable export controls on the shipment of firearms.
◦Items that appear to be prohibited weapons may be subject to additional checks and delays.
◦Sender's name and address must be visible on the parcel.

•UK - Allowed in the mail, see restrictions and packaging guidelines below:
◦Guns intended for sporting purposes - including Section 1 (e.g. hunting rifles) and Section 2 (e.g. shotguns) firearms, low-powered air weapons and their component parts may be sent in compliance with UK law and subject to domestic controls on the possession of firearms. Antiques, deactivated and imitation firearms are also permitted. When sending deactivated firearms include proof of deactivation.
◦Items that appear to be prohibited weapons may be subject to additional checks and delays.
◦Use 1st Class as the minimum service. Only low-powered air weapons are accepted by Parcelforce Worldwide and must be sent using express48 via the Post Office.
◦Sender's name and address must be visible on the parcel.