@kerley, from your link:
All non-flammable compressed gases are prohibited e.g. air bags; scuba tanks, carbon dioxide; fire extinguishers; neon and nitrogen
Shocks contain compressed gas. Even if you let the air out of the main chamber, the negative spring still contains compressed gas. I told my local post office it was a shock and they were OK, but is your average post office teller really qualified to understand and interpret these regulations consistently? More significantly, will them saying “OK” make the blindest bit of difference if someone else in a sorting office decides to confiscate your shock?
Based on their website I think Royal Mail are quite within their rights to confiscate your shock if they are so inclined.