Royal Mail regs seem to have got a bit more lenient lately. I had a 3L cylinder posted to me I bought off another forum 2 or 3 weeks back. They seem more concerned about gasses now, rather than the cylinder itself. The PO counter were quite happy when they were told it was an empty cylinder and the valve was open, and they were happy with that. The other cylinder he sold also got to its destination. The second half of the penultimate line seems to count. I assume a buddy bottle would need the valve unscrewed as it otherwise couldn’t be left open.

Gases, including flammable, non-flammable, toxic and compressed gases (including new, used and empty gas cylinders, ethane, butane, refills for lighters, fire extinguishers and scuba tanks, life jackets, nitrogen dioxide and carbon dioxide canisters including culinary foaming devices & soda stream)
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.
All toxic compressed gases are prohibited* e.g. chlorine; fluorine etc.
All non-flammable compressed gases are prohibited* e.g. air bags; scuba tanks, carbon dioxide; fire extinguishers; neon and nitrogen.
Empty cylinders that are sealed so that residual gas under pressure are prohibited, only those cylinders which are open to the atmosphere for inspection are permitted

*includes new, used and empty gas cylinders