But if a specific test agreed by the DfT with a specific airport requires an airtight bag it isn't unreasonable that there is an insistence on an airtight bag is it? I guess you could interpret sealable to mean airtight too couldn't you?

I don't know if any of this is true as above, it does seem plausible however. The bag that is supplied is the correct maximum dimensions isn't it? So if your stuff wont fit in it then there is possibly a bigger problem.

I have taken to leaving all my liquid items in the BAA supplied plastic bag which I keep within my robust toiletry bag. This makes it easy to empty into a new bag if required.

Since BAA isn't charging for the plastic bags (unlike some other airport operators) I really don't see any sinister plot here.