Just came across this which states the same thing I said earlier.
https://www.kingdomofairsoft.com/postage-guidelines
They also assume we can classify airsoft guns as 'Low-powered air weapons'
“Arms and Ammunition - low powered air weapons Low-powered air weapons (air rifles, air guns and air pistols), together with lead pellets and other airgun and airsoft projectiles, can be sent but are subject to the following conditions:
These items must be sent on an express48 service only
These items must be sent via the Post Office only
Enhanced compensation cover is not available”