hasn't there already been a precedent with the status of FSTA? Or - for example - the Securité Civile in France?
Maybe so. I am not a legalist and neither, dare i suggest, are you.
I do know that it was a major headache in the contract process and comes with significant limitations (of which i know no more detail, sadly!)
Post-WWII maybe, but it's still the major aviation statute in International Law and had been revised several times (in 1959, 1963, 1969, 1975, 1980, 1997, 2000 and 2006) so isn't that outdated!
As for HMG breaching it, you show your naivety - Article 3 clearly states that the Convention is not applicable to State aircraft.