Boris Johnson became prime minister last week after being picked by barely more than 92 000 members of his party who, as the Financial Times reported, make up a fraction of 1% of the United Kingdom's electorate.
Actually, this process was a lot more democratic than the ones used in almost all previous choices of Conservative prime ministers between elections. When Andrew Bonar Law died in office in 1923, it was King George V who chose the commoner Stanley Baldwin rather than the glamorous Lord Curzon to replace him as he thought it no longer appropriate for the prime minister to sit in the House of Lords. The king said he wanted his prime minister to face his (Labour) opposition in the House of Commons every day.
In 1957, when Anthony Eden resigned after his Suez catastrophe, the present queen appointed Harold Macmillan to succeed him after a marquess and an earl called in cabinet ministers one-by-one to ask whether they supported him or his rival, "Rab" Butler. Some of the ministers likened the experience to being "summoned to the headmaster's study".
Back in May 1940, Winston Churchill was appointed by King George VI on the advice of the resigning Neville Chamberlain after the alternative, Viscount Halifax, said he was not available, partly because he agreed that the prime minister should be a commoner and not a lord. Like Boris Johnson, Churchill became prime minister with a reputation near rock bottom. He was seen as lacking in trustworthiness, wisdom, and judgement. Only a year before, he had been the subject of a best-selling biography entitled Churchill, a Study in Failure: 1900-1939.
The first foreign leader to congratulate Johnson was Donald Trump, elected by 63 million American voters. This is a greater democratic mandate than Johnson's. But the strength – the defining characteristic – of the British constitutional system is that it makes for perpetual accountability, which the American system does not.
No matter the size of their mandate, or whether they are appointed between elections or after an election victory, British prime ministers hold office only for so long as they can command majority support in the Commons. Chamberlain thus resigned to make way for Churchill after his support collapsed over his failure as a war leader. He actually won a vote of confidence; what forced him out was a backbench revolt that cut the Conservative majority from over 200 to only 81.