He described how the most precious jewels were removed from the Imperial State Crown - worn at the state opening of parliament - so they could be kept separately in case of emergency.
It was buried below a secret exit on the orders of King George VI to prevent the jewels falling into Nazi hands.
It was such a closely-guarded secret that Queen Elizabeth, 91, who spent the 1939-1945 war at Windsor Castle for safety, did not know the details.
Alastair Bruce is one of the presenters of the documentary, which will be screened on Sunday. Telling her seemed strangely odd'.
The details were unearthed by Oliver Urquhart Irvine, the assistant keeper of the Royal Archives.
Thompson even had a couple of corgis accompanying her in her royal week out, the signature companions of the real queen. A trapdoor to access the hiding spot still exists today.
The documentary also features informal footage taken behind the scenes, including images of son and heir Prince Charles, then aged four, and his younger sister Anne playing underneath the queen's long robe. "Because if you did, your neck would break".
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The Queen still uses the Imperial State crown at some events.
"It's the sort of I suppose the beginning of one's life really as a sovereign", she said. "But once you put it on, it stays". I mean it's only sprung on leather.
Mr Bruce said the head has to be kept still when wearing it and the Queen agreed: 'Yes.
"Yes, and you can't look down", the Queen replies. Because if you did [look down] your neck would break, it would fall off. Anxious that the weight of the elaborate jewels at the centrepiece of her crown would injure her neck, she quips: "So there are some disadvantages to crowns, but otherwise they're quite important things".
It weighs almost 3 pounds, and Queen Elizabeth II jokes that you can't look down while wearing it or your neck might break.
In addition, she sipped on the Queen's favorite cocktail - rumored to be a combination of gin with Dubonnet - although Thompson was not quite sold on the drink.
Despite the country being in the grip of post-war austerity, a glittering coronation was staged on June 2 the following year at Westminster Abbey. That much was clear after he was with the Queen when she watched her coronation for the very first time, as part of the upcoming documentary The Coronation, which examines the 1953 ceremony on its 65th anniversary.