What if the US stayed neutral in WW2, leaving Britain as the sole democratic country remaining in Europe fighting against the Axis; yet the British still win the war, leading to them (along with resurrected France) dominating Europe afterward?
As a bonus challenge, have the USSR temporarily fall to the Germans, so it is truly Britain winning on its own, rather than just leading the Western Allies.
As a further challenge, have the United Kingdom become more imperialist after winning the war than before it.
Canada, Australia, New Zealand, etc. still participate in the war.