Maggie was elected to represent the constituency of Finchley, becoming PM due to being the Leader of the Conservative Party (the majority party after the election).
Blair was elected to represent the constituency of Sedgefield, becoming PM due to being the Leader of the Labour Party (the majority party after the election).
Neither one was *elected* as PM.
I know that some people do vote based upon the leader & therefore who they want to be PM, but regardless of that, it is not how the system actually works (although it would be nice if we did have a direct say in who became PM), as we do not choose our PM, not directly. OK so Brown is "unelected" directly... but technically, it's irrelevant. Whatever some people's intentions, the electorate does not elect the PM.
As for Kinnock > Blair & the landslide. It wasn't just a change in leadership when Blair took over, but a massive change in other ways too with the party when & after he took over. Plus don't forget that there was another leader in between the two, the late John Smith
. I do wonder what would have happened if he hadn't died... some think that Labour could have won still under his leadership, given the economy plus people being fed up with Tory rule (I do think he'd have been better than Blair...).