Why a fleet of Dunkerques would have alarmed Britain

How good were the Dunkerque-class battleships?

They were very capable; a French capital fleet rebuilt around them would have been sufficiently dangerous to provoke a much stronger British response in 1940.

The Dunkerques are described as capable, modern capital ships rather than inadequate compromises. A French battle fleet composed largely of them would have posed a much more serious danger if it fell beyond Allied control, giving the Royal Navy stronger reasons to act aggressively against French warships after the armistice.