On Royal Mail's prohibited items list:
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; Cigarette Lighters; 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; Carbon Dioxide; Fire Extinguishers containing such gases; Neon and Nitrogen.