If Britain retained western France into the Second World War

Could Britain hold a long-standing western French possession after Germany captured Dunkirk?

Probably not all of it, but a heavily fortified Brittany would have a strong chance of remaining in British hands.

The scenario gives Britain a long-held territory along the western French coast when Germany takes Dunkirk. Generations of conflict, including the Napoleonic Wars, would produce substantial fortifications across the enclave. Britain might still lose parts of the territory during the Second World War. Brittany is the strongest candidate to hold because its geography and accumulated defences would make it a formidable position.