I've been doing a bit of research on this lately as I'd been told the same - but its not quite correct in that Parcelforce Express 48 is not the only available service - it is, but only if your package is too large for the normal services...
The page you need is here: https://personal.help.royalmail.com/...stricted-items
And if you scroll down the page to "Guns for sporting use" and expand that it then says...
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.
The way I read that is that Royal Mail can accept low powered air weapons on the first class or RMSD service which DOES have extended compensation - BUT if its too large and falls into Parcelforce's service then only Express 48 can accept it and that is WITHOUT extended compensation.