......Aside from PF48, 'low powered, UK-legal airguns' can be sent via the Post Office using RM Special Delivery. Weights of up to 2kg are actually cheaper (£11.30) than PF 48 and you have compensation up to £500 included AND it's a guaranteed next day service You can see confirmation of this in the appropriate section of Royal Mail's Restricted & Prohibited Items list.

I've used it for air guns up to 2kg for years with no issues whatsoever, with the proviso that if the sender or recipient is in Scotland, N.Ireland, Eire and part of the journey would then be by air, PF48 is the better option.