Whether he was or was not using his AP is irrelevant as he should not have been where he was in the first instance - notably (1) in IMC (2) after dark (3) in France.
Issues (2) and (3) are really not significant as regulatory points. The report makes it clear that the only thing missing on (2) was that the physical piece of paper hadn't dropped through the door, and point (3) is a subtlety (almost certainly an unintended one) of FAR 61.3 the the vast majority of pilots and regulators neither know nor care about. French airspace is not more demanding than UK airspace. I'd be willing to wager that if it were not for the regular armchair-lawyer debates on Pprune and Flyer, and in particular post #36 in this thread, the BEA might not even be aware of the issue.
What is significant is that a pilot with no additional instrument training beyond the token amount in the PPL seemingly thought nothing of undertaking a night flight on top of a layer (albeit a fairly benign layer) of SC, knowing that at some point he would need to descend through it to land at his destination. This isn't the same as flying an ILS to minima on a dark and stormy night with a howling crosswind, but it does take a level of instrument competence. Is that, perhaps, another indication that Europe makes it too onerous to obtain the skills and privileges to do so legally?