"If I recall correctly, there were some technical constraints imposed by EASA but the national authorities had some leeway in interpreting/dispensing with them, which however required applying for this, but Air Atlantique never entered into a corresponding dialogue with the CAA."
I don't think this last is quite correct: as I understand it (and I wrote to various MPs and MEPs about the issue at the time, having gone into it in some detail) the CAA's interpretation of the requirements of the EASA Directive were so onerous (eg fitting an escape slide to the main passenger door of the DC-3s, if you can believe that), that AA rapidly concluded that it would be prohibitively expensive to make all the necessary modifications.
So probably another case of UK Civil Servants "gold-plating" a (silly to start with) EU Directive, especially as the Dutch, Swiss and Germans have all apparently found ways to keep their historic airliners flying (tho of course the Swiss are not directly subject to it, not being members of the EU-sensible people).
The above all said however, the news coming out of AA's successor organisation, the Classic Air Force at Newquay, very recently, is that there is, I think, renewed hope of a "deal" with the CAA, to water down their interpretation of the Directive, so as to make it economic for the CAF to modify their DC-3s to comply with it. I hope so!