M. A brings back a watch he bought in the UK to France, by air, value £250, they hold the necessary receipts.
£250 is under the €430 ceiling for French customs
It's their only purchase, is declaration still needed though no duty would be owed?
... is declaration still needed though no duty would be owed?
No.
Goods you have purchased or received as gifts in a third country*
When entering France, you may bring with you, in your personal luggage*, goods purchased or received as gifts in a third country* without filling out a declaration or paying duties and taxes.
Depending on the case, the value of the said goods must not exceed the amounts listed below: ...
From: https://franceintheus.org/IMG/pdf/trouble_free_travel_with_french_customs-2.pdf
In the case of entering France (or other EU countries), you are only obligated to declare items if you exceed the duty free limits.
There may of course be exceptions, but I would assume that this applies to most countries and not just the EU.