Large, yes. Powerful, from a geopolitical stand point. Compared to the Nazi war machine? Barely had a chance. What kept Hitler out of Britain was the sea separating Britain from mainland Europe and the fact that Hitler's main desire was to conquer the Soviet Union.

The truth is neither France nor the British empire were ready for the conflict. Years of appeasement to avoid another war was coupled with a lack of defense spending. Almost all of the French and British military were outdated. If Britain had been on the mainland their fate would have been the same as France. Mainland Britain would be part of the third Reich and Britain would have been allowed to keep their overseas colonies provided they conceded to Germany.

Of course it's also possible they would have "carried on the struggle" as per Churchill's famous never surrender speech. Thankfully, we never found out.