Here is all you need to know about the use or misuse of cylinders:
https://www.calor.co.uk/news/illegal-handlingscrapping-of-calor-gas-cylinders/
That is the legal stuff.
Whether people adhere to the letter of the law is their business, but should they be caught out, don't bother moaning about it, including those who buy/take/whatever cylinders from other sources and have the misfortune to have a cylinder fail on them,,,,,,with all that involves.
And that is enough said about the situation as this topic has cropped up almost every year with the exact same kind of responses.
The original topic of Gas for Winter is simple, Propane for Winter, Butane for Summer, or Propane all year and save messing about with pigtails or different regulators.