Careful before giving advice on these kinds of issues buddy.
Straight from Parcel Force: "
All firearms, other than low-powered air weapons sent within the UK (air rifles, air guns and air pistols), including imitations and antiques; paint-ball or toy guns; taser guns; components of firearms and all other items similar to, or resembling, the foregoing are prohibited."
- https://www.parcelforce.com/help-and-advice/sending/prohibitions-and-restrictions
That's pretty clear that you can't send something that would be defined as a "Realistic Imitation Firearm".
You can send airsoft ammo, but not RIFs. A manager at a post office also said I could send airsoft gas mags as long as they were depressurised, but this could also fall under the "components of firearms (resembling)".
Since companies these days do everything they can to wriggle out of paying up, I think you're going to have a tough time. You could have possibly made a claim if you had declared it as an air weapon (which is described as
a gun which fires pellets using compressed air), but to be honest, you're probably buggered. I use UPS, and it's a pain in the arse and the service is terrible, but as far as I know, you can still send RIFs.