I just checked with the post office as I was sending a letter - they confirmed that sending gas magazines, as long as theyre empty, is fine with Royal Mail.
However, during inspection, they might not be able to tell if it's depressurised or not, so to make sure they're not seized and destroyed, include a sticker on the package that states "Depressurised/Empty Vessel". Also, if it resembles a weapon part, also having a note inside that says "Theatrical Prop" or words to that effect will also be a good idea.
As long as its depressurised, as they put it, it's no different to sending a thermos flask.
UPDATE:
The detailed information on royal mail's prohibited items list includes empty gas cylinders, so whether this applies to magazines too is still a mystery, and appears to be luck of the draw on whether it's inspected. I would think your only safe magazine to send is a CO2 mag.
Airsoft Guns however ARE allowed to be sent Royal Mail (again, a package note would be a good idea), and Airsoft ammunition is specifically stated as allowed. However it must be sent at the post office and presented at the counter (I presume they put a "restricted-verified" sticker on it or something like that).