Several issues, first is just finding someone who will carry an airgun, second you are relying on the seller to package it but if you do the arrangements I think it's you who will bear any risk as the one paying for the service.
RFD-RFD will cost more, probably £25 at each end but is legal and usually the sending RFD will pack the rifle himself, so he see's it before it goes to confirm the condition, this is what I'd choose.