I am afraid you will have to do your own homework on this one, unless somebody else can offer the statute under which HMG can enact legal instructions to an airline or any other body. I have provided the information letter that I have (stored), which should provide the clues to anybody in the know.
The letter states that HMG directed (legally instructed) UK airlines in November 2003. It wasn't the civil servant who wrote the letter, or me. All UK airlines are aware of the directive, and as far as I know have incorporated the legal instruction into their respective operations manuals. As such all relevent crew members should also be aware of the directive.
If you want to test the defence that such an instruction is non-existent, unlawful, or ineffective in it's construction, let us know how you get on. I would be interested in knowing the result.