this is the reply I had today from Parcel Force. please read all of this.
I can confirm it is possible to send an air-rifle through Parcelforce. Due to recent changes some new restrictions do apply. Please see these below:
These items must be sent on an express48 service only.
These items must be sent via the Post Office® only (not via any other channel e.g.www.parcelforce.com
, call-centre, reseller or Parcelforce Worldwide depot).
Enhanced compensation cover is not available (low-powered air rifles, air guns or air pistols and lead pellets and other airgun and airsoft projectiles will only be covered up to the maximum amount of inclusive cover for express48). £100 max
Note: Low powered air weapons are defined as air rifles, air guns or air pistols capable of discharging a missile so that the missile has, on being discharged from the muzzle of the weapon, kinetic energy not in excess, in the case of an air pistol, of 6ft lb. or, in the case of an air weapon other than air pistol, of 12 ft. lb. Any air rifle, air gun or air pistol which uses or is designed or adapted for use with, a self-contained gas cartridge system is a prohibited weapon and cannot be sent. unquote.