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.