yes surface mail seems to be the safest but do not put Co2 capsules in with it .
parcel force 48 and special delivery are the only ways to send it i think.
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, and presented at the counter
Enhanced compensation cover is not available
https://www.parcelforce.com/help-and...d-restrictions
i'm sure if i have the above wrong some one will advise you better.