You guys did take out the French fleet to prevent it from falling into the hands of the Nazis during WW2.
It's a pity we can't sink French fleets more often, just like we used to in the good old days. Just sinking French ships every so often when the French are stupid (or cowardly) enough to leave them available to the enemy for capture is not often enough for my liking. British males are denied a good war against the French nowadays.
Who DECLARED war?
War was only declared because Germany invaded Poland. It was that act of Nazi Germany which finally started WWII.
Moreover, the French would have become German toadies,
They ARE German toadies. The Germans prop up the third-rate, socialist French economy for a start.
and the British would have been very much alone in the world.
Not when they had a vast empire. In fact, a defeat for Britain in WWI could have meant the British Empire lasted for longer than it did, so a British defeat could have been not all bad.