Did you send it pressurised? I suspect they wont take too kindly to that if you did. A quick look suggests that no gases can be posted. I imagine you fell foul of number 3.
Gases
Gases that are compressed, liquefied or dissolved under pressure, permanent gases which cannot be liquefied at ambient temperatures, liquefied gases which become liquid under pressure at ambient temperatures, dissolved gases which are dissolved under pressure in a solvent.
1. All flammable compressed gases are prohibited e.g. blowlamps; butane; lighters and refills containing flammable liquid or gas; ethane; gas cylinders for camping stoves; hydrogen; methane and propane.
2. All toxic compressed gases are prohibited e.g. chlorine; fluorine etc.
3. All non-flammable compressed gases are prohibited e.g. air bags; scuba tanks, carbon dioxide; fire extinguishers; neon and nitrogen.