Two 20th-century titans: Winston Churchill and David Lloyd George (right) at the memorial service for Arthur Balfour in 1930 (photo: AP/Press Association Images)

Not difficult to see why Boris Johnson jumped at the publisher's request to write a life of Churchill. It was an excellent opportunity to show why, on the eve of a general election and Boris's bid for the Tory leadership, he himself has a touch of "the Churchill Factor". And in any case, Churchill has now joined the trio of subjects, making it a quartet, who can be relied on to do superbly as biographical choices, the others being Napoleon, Byron, and Mary, Queen of Scots.

Not that Boris has much in common with Churchill, beginning with hair — or lack/plenitude of it. The Dukes of Marlborough found it hard to hang on to their hair — the founder wore a wig, even at the battle of Malplaquet, I believe — and Churchill lost his early. It had been red, or rather "the colour of a bronze putter", as Lord Curzon sniffily put it. Churchill was not sorry to see it go, red-haired leaders always having had trouble with their party, Peel and Baldwin being other instances.

Then again, Churchill, though a Tory Democrat like his father, and not above stooping to Tory demagogy on occasion, never found it easy to affect the common touch. Born in Blenheim Palace, his natural habitat was (to quote his bitter opponent Nye Bevan) "Aristotle Onassis's yacht". Monte Cristo cigars sprung naturally to his jaws, and glasses of Veuve Clicquot (preferably the 1928 vintage which he rated "the best ever crammed into a magnum") to his lips. He never wore a trilby or a pork-pie, rode a bicycle, smoked a Woodbine, or downed a half-and-half. Boris does all of these kinds of things, effortlessly.

There is also a sharp difference in mental calibre. Churchill was a genius, at times, with a natural wit, profound long sight, a disturbingly acute sense of danger and wonderful word-play. But a super-high IQ? A first class brain, a natural for a double-first, an All Souls Prizeman? I wonder. Anthony Kenny, sometime Master of Balliol, supposedly Oxford's cleverest college, told me: "In all the years I was Master, Boris was by far the most intelligent undergraduate." No one would have said this of Churchill, though Dr Jowett would doubtless have spotted the other gifts.

Churchill could be, and often was, a bad judge of character, as his most loyal critic, Clementine, was to lament. He stuck to the lunatic Admiral Fisher. He backed the scientific judgment of "the Prof" (Frederick Lindemann, later Lord Cherwell, who underestimated the potential of nuclear weapons). Bad picking was at the root of the Dardanelles disaster. Trusting Beaverbrook often got him into messes. Glamorising King Edward VIII, failing to spot he was no good and never would be anything but a shit, led to the only occasion when Churchill was physically howled down in the House of Commons, and might have ended his career at a blow. Boris does not make these kinds of mistakes.

Reading his book, noting his admiration for Churchill but also his selection of the Churchillian qualities he chooses to praise, I am led to the conclusion that he has most in common not with Churchill but with Lloyd George. It is significant that there is comparatively little mention of Lloyd George in The Churchill Factor. It is as though Boris is subconsciously avoiding the subject. But anyone who studies the history of Britain in the first half of the 20th century cannot avoid answering the question: who was the greater man, Churchill or Lloyd George? I used to discuss this point, and their relationship in general, with Lord Boothby. He had known them both, in some ways very well, even intimately. He was never in any doubt that Lloyd George was the greater man. I don't think I agree at all. But the point is worth considering.