To clear up any confusion on this subject, caused by Post Office staff thinking Royal Mail has discretion on whether it can carry airguns, please note that the RM regulator Postcomm ruled clearly in our favour in June 2005.
Royal Mail is stuck with this decision like it or not. They must carry legal airguns, properly packaged, as long as they meet the size requirements of Standard Parcels and Special Delivery.
A one-page summary of the ruling for you to print out is <1> and the full ruling (PDF format) is <2>.
It makes sense that we do our bit to make sure all Postmaste