Well, the British had betrayed the French by signing a naval agreement with Germany, a complete violation of the treaty of Versailles. The British also betrayed France by sending only 500 000 men for the battle of France against Germany. The British expected the French to be cannon fodder and were wrong. They had no legal right to attack the French fleet, it was unprovoked as Edric said, unless losing the war is a provocation.
except of course, having not surrendered, it's difficult to see the British as having lost the war, which is what happens when a country surrenders, not when it carries on fighting