Hi kazz welcome back to SF
I doubt it really matters to the Fisc to be honest. A couple of our UK accounts were opened in the 70’s so I just made up a date that was reasonably representative of when they were opened from memory.
Don’t be unduly concerned. The fact you are reporting the accounts is the important thing and the account number/branch detail is the bit from which they can make any search - if they are minded to do so.
From an International money laundering perspective, when you declare to HMRC that you are resident in France, they will report any account activity as required.
As to completion of the 3916, the guide I provided for online submission covers what to put on the form and where. It applies for the paper version too.