Here's an article from 2003 by Sir David Owen--a Brit who is both a physician and an ex-Minister. Lots of goodies in here.
On the Shah of Iran:
Although there is no evidence that the CIA or MI6 ever knew that the Shah had leukaemia while he was in Tehran, there is some evidence that the KGB knew….
[The extent of his illness was hidden from the Shah himself, but he knew enough to keep his illness secret]. If Washington knew he was ill, he could no longer expect the same unqualified American support he now enjoyed. He would be deserted by his allies’.2 That judgement was correct.
… in retrospect what he needed was to be told what to do and virtually forced to take treatment in Switzerland. If he had done so, the Revolution in Iran would not have taken place in the way that it did, President Carter might have won a second term, and certainly the history of the Middle East would have been very different.
On Idi Amin:
Some physicians hazarded the opinion in the newspapers that he might be suffering from general paralysis of the insane. That diagnosis, however, has never been substantiated, and is unlikely, since he is still alive today, having fled to Saudi Arabia after the Tanzanian Army invaded Uganda in 1979 with the full encouragement of the British government.
What Mugabe had within him was an odd combination of Jesuit Catholicism and Chinese Communism. After an amazing period of reconciliation which surprised everyone and was unsurpassed by anything anywhere else in the world, Mugabe within a few years used North Korean troops in a brutal suppression of the population in Matabeleland, and in recent years he has presided over the ruination of his country. Commentators today refer to Mugabe as mad: I doubt if he could be so diagnosed, but he is undoubtedly acting evilly, and ought to be removed as President.
On Francois Mitterand:
Mitterrand's early view that there should be the utmost transparency about his health, following the death of President Georges Pompidou from an undisclosed blood cancer, changed when he became ill. Mitterrand told his doctor that his illness must be considered a state secret.
…. Was President Mitterrand entitled to suppress the knowledge of his cancer of the prostate? Probably yes, for in 1981 he had only just won the Presidency for the Left in France after a long period of Centre Right dominance, and his mental function at that stage was totally unimpaired. President Mitterrand had much less justification for withholding news of his cancer before putting himself forward for a second term in 1988.
I spoke to him on a one-on-one basis in 1978 in the Foreign Office, and again in the White House in 1986. It was very hard to assess his mental capacity at the best of times, because of a self-confident ignorance on some important matters and his charming gift of self-deprecation. Reagan was a strong-willed leader, but content to concentrate on presentation. His management style was to focus on a few big issues, which he then excelled at simplifying.
… [Given the later progress of his Alzheimer's] It is hard to believe that he was not suffering from some cognitive impairment while President.
On Churchill, who had one health crisis after another, and always drank a ton, but kept bouncing back:
Churchill would have been forced out after his stroke in 1953 but the key politicians who knew his true condition after the stroke never acted. Anthony Eden was still sick. Harold Macmillan had also been ill and was not yet in a powerful enough position to strike, and Rab Butler was not ruthless enough to insist on being made Prime Minister, although he probably could have done so. Churchill eventually retired, far too late, in April 1955 and died in January 1965 after his last and most severe stroke. [This helped bring about the Eden-as-crazy-PM situation in 1956].
Owen concludes that Anthony Eden's behaviour as British PM during the Suez crisis was probably affected by barbituates, but not Kennedy's, even though Kennedy had similar drug problems, during the Cuban Missile Crisis. His general conclusion is that candidates for office should be subjected to medical scrutiny, but in the case of someone already holding office, especially in a crisis, caution should be shown in publicizing bad news. Churchill had a heart attack in 1941, right after the U.S. had decided to enter the war.