Maybe he meant “The British Empire, alone”.
In a sense that’s true, when it comes to organized government. Of course, there was a French government in exile, and a Polish government in exile, and a Dutch government in exile, and the French and Polish and Dutch and other resistance movements began immediately. They were not very strong, but they were in the field fighting.
Britain, Canada, Australia, New Zealand, South Africa, the Indians, etc. were all in the field, under the grand banner of the British Empire.
China was in the field too, of course, but that was on a different front.
If we said that the only organized major national fighting force in the field in Europe still fighting Hitler in 1940 after the Fall of France were the forces of the British Empire, that would be true. Canadians would be included prominently in that.
To say that the British, qua British, were the only folks facing Hitler alone during that time period would be false. But I wonder if the intent was to ignore the others, or it was just an oversight.
Up thread, for example, I called the Scots and English “English”, but that’s not really what I meant. What I meant was that they’re both Anglo-Saxons, and THAT is true. So are the Irish, in that sense. And the Americans. If by “British forces” we mean “British Empire forces”, it’s a different case.
By the way, I have a question. Perhaps someone can answer me. Back in 1940, could the Americans trade directly with India or British Africa, or were there all sorts of trade restrictions in American access to those markets sown up by British Imperialism? And if the latter, what was the strong American interest in fighting for the British Empire (other than that the bad guys were bad)?