This is maybe too late but as they say "better late than never" !!
You are not allowed to detect on all the beachs in Normandy where D day took place !! It is illegal !! as in certain parts of France (Oise, Aine and a few other parts !)
Generally you are allowed to detect beaches, but not in day time during summer when people are on beaches and normally I would recommend that you ask the local town hall as some towns do not allow the use od detectors !!
You do not need a permit to detect beaches, and I should imagine that if you rae foreign (to the French !!) they will not cause you too much pain (apart from on d day beaches that is !!) !!
Do you have any particular area in mind ??
Yep sure would be a nice place to be right now !!
But must say I prefer quieter places such as brittany .....
Less sun, less people and nice food too !!