That last sentence is the reason why there was a change in the legislation.
The link below has some info and also links to other threads on the subject.
Don’t accept anyone else’s previous experience as valid for today. The restricted and prohibited lists should be checked each time when sending a cylinder, so you will have to pick a couple and check the lists.
The thing that matters is the methods the carrier uses across the whole journey. Legislation covers what and how goods can be carried depending on surface mail, airmail etc. you can or assume that a package from mainland U.K. to mainland U.K. will never go by air.