I think it's fairly straight forward, Norm.

All guns must be securely packed.

As long as a PCP or CO2 low powered air pistol or rifle has no air/CO2 in the cylinder, it's fine, and CO2 cartridges must not be included with the gun.

First class can be used, but for full insurance cover Royal Mail Special Delivery should be used.

PF48 is an option, and for guns over 60cm in length, it's obligatory, and there is a maximum of £100 insurance/compensation.