I've sent a few.
The terms on their website you need are the following:

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

I usually open their terms and conditions on my phone before hand, just in case I need to show them its ok