People have been told both yes & no, "pressure vessels" are prohibited either full or empty, But the rules equally say airguns can be sent & do not state PCP's are not included.

IF you do then mark the outside "surface mail only" - RMSD usually goes by air at some point, but unaccompanied firearms are prohibited by the CAA so a "gun shape" (on Xray) will be refused flight & it's highly likely RM will then send it to Belfast to be opened & inspected, at which point it's pot luck.