Let's try to make a comparison between the situation in the US between 1788, when Washington was first elected president, and today, and that in Britain. Comparison is made difficult by the fact that the UK's political system is quite different from the US's – there is no such thing as a fixed term – whether four years or otherwise – for Prime Ministers, and it's perfectly possible (and has sometimes occurred) for PMs to be forced to call an election, and hence potentially end their own term in office, because they have lost a motion of no confidence in parliament. Moreover, British political parties have shown themselves perfectly willing to depose an incompetent or fatally unpopular leader in the middle of a term in office if they conclude that he or she has become a liability at the ballot box, a fate that famously befell Margaret Thatcher.

So, with that said, let's take a look at the "raw scores."

In the 229 years since the US elected its first president, the UK has had 61 prime ministers, at least if we count using the US style whereby PMs who lose their job but return to it after another administration has taken office are counted more than once (for example, Gladstone served as PM on four separate occasions, Derby and Salisbury on three). Of these, only one has been assassinated: Spencer Perceval, who was shot in the lobby of the House of Commons in May 1812 by a disgruntled merchant named John Bellingham. Bellingham had been imprisoned in Russia for most of the period between 1804 and 1808 as a result of a dispute over a debt, and had attempted to press claims for compensation with the British government. He seems to have blamed Perceval personally for his failure to secure recompense.

Four Prime Ministers died in office during this period. They were:

William Pitt the Younger (1804, after 21 - non-consecutive – years in office) – probably of a stomach ulcer

William Cavendish-Bentinck, 3rd Duke of Portland (1809) - after an unsuccessful operation to remove kidney stones

Spencer Perceval (1812) - assassinated

George Canning (1827) – pneumonia

Among "near misses" we might note Henry Campbell-Bannerman (died 1908, 19 days after resigning as PM) and Andrew Bonar Law (1923, resigned after being diagnosed with the throat cancer that killed him 6 months later).

So a total of 6.5% of British Prime Ministers have died in office, compared to almost 18% of US presidents.

The difference between the two groups in terms of age when leaving office (for whatever reason) is almost imperceptible - 63.1 years for the prime minsters, versus 62.2 years for the presidents. So I think we need to seek the reason for the discrepancy in the difference between the two political systems. The British system allows for a PM who is seriously ill to stand down and be replaced by another member of their own party, thus retaining the party's grip on power, in a way the US system does not encourage. I believe this is the main reason for the difference in the proportion of men and women dying in office, as is perhaps backed up by the fact that no PM has died in office since 1827 – remember also the existence of the pair of British PMs who resigned their posts when terminally ill and died very shortly thereafter.

I'd also like to add the reason I asked this was, in comparing the us to Canada, there's only been 1 prime minister that has died in office John Thompson. He died of a heart attack at the age of 49 on December 13, 1894. I didn't even realize there had been a Canadian pm to died in office. Granted there's 90 years less of Canadian history that's still only 1 in 25.