Passport to Pimlico

A comedy film made by Ealing Studios in 1949, starring Stanley Holloway and Margaret Rutherford and directed by Henry Cornelius.

When an unexploded WW2 bomb is accidentally set off in the working-class Pimlico district of central London, documents are uncovered proving that the area is part of the Duchy of Burgundy and thus foreign territory. The locals are glad of the opportunity to declare independence from the bureaucracy and austerity of post-war Britain. The British government, in response, sets up border controls and export restrictions in an attempt to starve the Burgundians into submission. Hilarity Ensues.

The film was a big influence on the 1959 comedy The Mouse That Roared.

Tropes used in Passport to Pimlico include:

Alliterative Title.
Anvilicious: The film was made at the time of the Berlin Airlift and audiences would have made the connection between the siege conditions of the Burgundians and the real experience of the Western Sector of Berlin.
Dramatic Thunder: At the very moment Britain and Burgundy are reunited once more, as marked by the chimes of Big Ben, there's a clap of thunder and rain pours down on the celebration banquet.
Heat Wave: The lunacy takes place during an uncharacteristic London heatwave. Only when the more traditional rain returns is normality restored.
Obstructive Bureaucrat: The British civil servants negotiating with the Burgundians aren't very competent. Usually it's themselves they obstruct. War-ravaged Brits had had enough of officialdom, which didn't let up during post-war reconstruction, and would have been glad of the opportunity to have a laugh at the officials' expense.
The Siege: The British government cuts off supplies of food, water and electricity but the Burgundians stand firm with the help of the stock held by the pub, a water pipe attached surreptitiously to a fire hydrant on the British side of the border, and food parcels thrown over the border by sympathetic Londoners.