Tommy Lascelles (Paul Brooke) is depicted as the Private Secretary to Bertie from the start of his reign, and certainly from the beginning of the war. In fact, though he had been an Assistant Private Secretary since shortly before King George V's death, he was only promoted to the full role (directly dealing with the King and his boxes, for example) as late in the War as 1943. He remained at post for the rest of the King's reign, several years after retirement age, and into Elizabeth II's first year as Queen. It's best to think of the role as combining two real-life people (Sir Alec Hardinge and Sir Alan "Tommy" Lascelles) into one.
Denis Lill (Clement Attlee) previously played King George VI in The Gathering Storm (1974) and his paternal grandfather King Edward VII in Lillie (1978).
In a career rarity, Robert Hardy appeared in the film but not as British Prime Minister Winston Churchill. Instead, he played Churchill's counterpart, U.S. President Franklin D. Roosevelt.
The film takes place from 1922 to 1952.