It's a UK industry standard, that is then used by the UK HSE to underpin a number of UK Regulations in respect of Flammable Gasses.
The unlawful bit comes when companies who supply LPG allow customers to fill their out of date ( so not conforming to the Regulations) refillable cylinder - those companies would then be in breach of UK HSE law.
If they do not want to break the law and risk being prosecuted or having the gates to their business locked up by the HSE, then suppliers of LPG will refuse to fill your 18 year old cylinders and inspectors carrying out Hab Checks will check all of your LPG cylinders, hoses and regulator for being in date. They would bring to your attention your 18 year old equipment.
So it's not unlawful for a person to have out of date LPG cylinders, but getting them filled at a law abiding LPG supplier would be a little problematic.
Thank you … at last a sensible answer..