Francis Urquhart's catchphrase "You might very well think that; but I couldn't possibly comment" (meaning "Yes, but I'll deny everything if challenged") has become part of the English political language. "Urquhart's Avoidance" is as widely known as real-life political euphemisms such as "he wants to spend more time with his family" ("he's been sacked or resigned in disgrace") and "he's been economical with the truth" ("he's told blatant lies to save his own skin").
By complete chance, the first BBC showing of the series exactly coincided with the real life Tory leadership contest and the removal from office of long-running Prime Minister Margaret Thatcher.
Ian Richardson said he based his performance of the scheming Francis Urquhart on the way Shakespeare portrayed Richard III.
The French half-glasses which Urquhart wears were bought personally by Ian Richardson, as he thought they would be perfect for his character. He didn't send the receipt to the BBC after the series as he wanted to keep them.
John Major's then-running campaign for the leadership of the Conservative Party, following Margaret Thatcher's ousting, actually downed tools when the series aired in order to watch it.