You can send air weapons through normal post too - it's a restricted item, not a prohibited one - it's even on Royal Mail's website
https://personal.help.royalmail.com/...detail/a_id/94
The only things prohibited are Section 5 and illegal weapons.
I'm not sure where the PF48 thing came about?
I've recently received 2 rifles through PF and both arrived perfectly safely, mostly as a result of the good packing by the sender!