The state of Britain’s finances as a result of its banking fiasco are known. The state of France’s finances and their possible exposure to its banking fiasco are unknown.
As our old friend Rumsfield said about known knowns and unknown knowns. The markets work (when they are working) on the basis of information, known as known knowns.
The unknown unknowns are now known as black swan events.
To that you can add that the BoE has shown great flexibility in its dealings with banks (after Northern Rock). It has operated as a fully functional lender of last resort.
The ECB are the problem with French banks as far as the ratings agencies are concerned (i.e. with the link between banks and state).