Forfoxake, I knew you were a bit further north when giving this advice! ;-) I agree - any distance over cold water, dry-suit + lifejacket + life raft + PLB + flight plan as an absolute minimum.
PPR doesn't (normally) exist at airfields in France for GA. This isn't PPR, this is a customs form (like the GAR in the UK) that
IS required to be filled out and sent at least 2 hours prior to planned arrival. Different regions and different airport have different arrangements with local police / border-force but it is a requirement for it to be filled out and sent if coming / going to / from a non-schengen country. I have heard that most are now trying to have a similar forms and systems but it is taking time.
Passports - I agree - but I wouldn't want to be caught-out crossing an international border without a valid passport, folklore says that the pilot is responsible for carriage of and validity passports, I am not sure how true that is, but I wouldn't want my friends or family ending up in a cell for the night (or more)! I also don't want to be the person who gets half way across the Channel and diverts back to their home airfield and having stated on the radio that the reason for the diversion is that they "had left the passports on the planning table in the clubhouse" - true story on London Info a few months back!