It's a parliamentary system over there, old son. The leader of the opposition, who regularly excoriates the current PM in the well of the House, is often a former PM. And you win the office of PM by being the leader of the party that wins Parliament. Whole different thing.
There I've done it. I've splashed my ignorance all over the Internet.
Right, opposition leader can be former PM. Right now, Tory leader is Michael Howard, but the criticism from Thatcher is not at all a breach of etiquette. Not at all analogous to Clinton's White Trash ways.