Meet the Second World War veterans who felt compelled to fight against their homeland.

There is that wonderful line in Kubrick's Dr Strangelove when the American president admonishes the Soviet ambassador and one of his Air Force generals for wrestling over a spy camera: "Gentlemen! You can't fight in here – this is the War Room."

It was Ken Adam who came up with the design for that simple but unforgettable set: a dark, oppressive, cavernous chamber dominated by a huge, circular conference table and giant, glowing wall maps marking the approach of nuclear Armageddon.

Adam, 88, is the only German to have served as a pilot in the Royal Air Force during the Second World War. As a Sergeant Pilot in 609 Squadron, flying Typhoon fighter-bombers on low-level strikes across northern France, he would have been put in front of a firing squad if captured and identified. Jewish-born but still the holder of a German passport, he was to the Nazis a traitor.

Some 10,000 men and women from Germany and Austria, Jews and other opponents of the Nazi regime, fought in British uniform. As "friendly enemy aliens" they could not be compelled to join up. All were volunteers, representing almost one in eight of the 78,000 German and Austrian nationals who fled to Britain before September 1939.

The King's uniform did not confer British nationality. Those who wanted to make Britain their permanent home were granted passports only after the war. Hated and persecuted in their homeland and treated with suspicion in their adopted country, they lived out the war in a kind of limbo, uncertain as to what the future held. What drove them was an absolute detestation of Nazism.

Sir Ken is one of a handful of veterans featured in Churchill's German Army, a documentary chronicling this most curious of Allied contingents. It is 75 years since he left Germany but he still speaks with a soft German accent.