I've had both yes & no to PCP bottles cylinders,
Best I can say is that unless/until they state "guns for sporting use" does not include PCP's, it says it is legal to send "airguns".
So do everything correctly, empty it, declare it as a "gun for sporting use" as per their rules allow sent by PF48, and there should be no reason for it to be further stopped/inspected.

If you are so stupid as to lie about the contents or use a cheaper service to save a few pence, don't moan if it goes wrong.
otoh if you do things properly & something goes wrong you have sent an airgun as per their written rules state they accept them.