That would soon be rectified. RAF Coastal Command would be directed to sweep the Channel, seeing what was in the French ports and what was heading North.
Well, since you mention it...
Not much at all! :P
Remember that it is longstanding French policy, since before WWI, not to focus significant forces in the Atlantic Fleet, instead depending for the most part on allied forces (Atlantis, Russia, Britain, and now recently Germany) to dominate these theaters. The sole French force the British would be most concerned about - the
Force opérationnelle Atlantique Nord, headquartered in Brest, with one fleet carrier and two capital ships - is currently visiting Cam Ranh Bay. The British should be pretty well aware of this deployment, since it was published in the newspapers, and the force had to pass either Panama or Suez on their outbound leg. Their absence means the French Atlantic Fleet has no surface ships of destroyer size or larger north of Dakar, with the exception of the
Forbin-class destroyer
Savorgnan de Brazza, which is two months into her shakedown cruise, and
Le Beautemps-Beaupré, which completes on April 15th.
Of the coastal commands, some details:
-- 1st Région Maritime, HQ at Dunkerque, seems to have all four of their CS-27 class submarines at sea. The rest of the force - ten 120t subchasers and four
La Combattante II motor torpedo boats, is either pierside or derping around in the Manche, conducting the normal coast guard sorts of patrols.
-- 2nd Région Maritime, HQ at Brest, has five of their six submarines at pier;
Pégase is at sea somewhere (possibly in the Manche, more probably in the Mer Celtique). Four of the eight minesweepers are running around in Quiberon Bay, while the rest are in port.
-- The 5th Région Maritime, HQ at Lorient, appears to be pretty active, with two of their four submarines at sea, and five of their submarine chasers undertaking coast guard patrols - but their responsibility and zone of action is south along the coast of the Golfe de Gascogne.
The Forces Sous-Marins, HQ at Lorient - with twelve
Emeraude-class fleet submarines and the two
Roland Morillot-class submarines, appears to be 100% in port (alongside the 4th Fleet Submarine Flotilla, the sub wing of the FO Nord, which stayed back from the voyage to the Pacific). If the British have bothered establishing a baseline of behavior, it's extremely unusual for the French to have all twenty of those submarines to all be sitting in port at the same time. Usually at least four to six of them are at sea.
No warships at Cherbourg aside from
La Combattante herself, which is continuing to field test her gas turbine engines by tearing up and down the Manche.
The British MIGHT note that four
Étendard-class destroyers of the 6th Flotilla Torpillieurs, normally homeported in Toulon, are currently derping around in the Alboran Sea, sailing for Tangiers in company with the naval tanker
La Vestale - the old one, in one of her last cruises. If asked, the French naval attaché will point out that they annually hold Exercise Orion, a small antisubmarine exercise, in conjunction with the Irish Naval Service around the month of May. With FO At-Nord in the Far East, the exercise this year has fallen to the first division of the 6th Flotilla Torpillieurs, out of the Mediterranean Fleet.
It's almost like the French got tipped off about the big German exercise a few weeks in advance, and are sitting around looking angelic.