no its not against the law buddy
here ya go buddy this should help!
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.
The Postcomm press release is HERE and the full ruling is HERE.
It makes sense that we do our bit