How To Become Prime Minister?

Here's a thing that occurred to me while watching "Margaret", the BBC drama about the downfall of Margaret Thacher. Since the war, if you want to be Prime Minister, you are better off hanging around as a senior person in the Government, than by being leader of the opposition.

To see what I mean. Since the war, the following leaders of the opposition have become Prime Minister:

Attlee, Churchill, Wilson, Heath, Wilson again, Thatcher, Blair.

Of course second Wilson, but also Churchill, had been Prime Minister before being leader of the opposition. Leaving:

Attlee, Wilson, Heath, Thatcher, Blair.

The following have succeeded a sitting Prime Minister who has left for some reason.

Eden, Macmillan, Home, Callaghan, Brown.

Indeed Churchill's wartime premiership started by taking over from a sitting Prime Minister of his own party.

So both lists are length 5, so maybe I'm overdoing it.