Extract from the Dept. for Transport guidelines:-

https://assets.publishing.service.go...nce-note-9.pdf

Private Carriage of Oxygen Cylinders
7. Whilst the carriage of oxygen cylinders by private individuals is not covered by
ADR or CDG as they are fully exempt, some simple safety precautions should
be taken, such as
• Do not smoke whilst carrying cylinders;
• Check cylinders for obvious signs of leakage, for example hissing
sounds;
• Keep cylinders out of direct sunlight;
• Do not expose cylinders to high temperatures;
• Secure the cylinders properly so that they cannot move in transit (some
cylinders may be transported horizontally or vertically; however you
would need to check with the supplier).