We lived off a lump sum (no interest) when we first moved to France, with no problems.
Some funds here in Credit Agricole cheque account and some in UK cheque account. Transfers made between the accounts, were not income and were not taxed. But the Bank Accounts (and hence their balances) were fully declared.
Why not go to your Tax Office now and talk things through with someone in the Income Tax department. They will be rushed off their feet during the “official” Declaration period… so they should be calmer at the moment.
As you will be making your first Declaration (and a late one at that)… you may well need quite a bit of paperwork… which is why I recommend you go and speak with them now. They are helpful folk and they will advise you and put your mind at rest.
Complete disclosure is essential…but this Declaration will get you into the financial part of the French System…
If, as you say, you have Nil income… you will not be taxed…(unless, as David says, you are super wealthy )
If/when you make transfers above a certain amount (can’t remember the figure) from a UK account to a French Bank Account… you will almost certainly have to prove to your French Bank…just where that money has originated from.