I suggest the British consult with the French, as they are the undisputed masters of speedy withdrawals when faced with an invasion.
The French took massive casualties in World War One, and held the line. I think they lost 1.5 million men. Britain lost 900,000.
The Brits are already experts at it. They surrendered Singapore to a Japanese Force 1/3 their size. This upended conventional military thinking that believed the attacker needed a 3 to 1 numerical advantage.
They also bravely ran away from the French flank at Dunkirk, and went home to tell everyone about the French.
Both were probably justified moves, but the point is, but for the channel, England would have had the French occupation experience, complete with a British Vichy government. So they really should tamp down the French surrender meme.