[West Ender]

The position of Deputy P M is a job in itself. It means he can deputise when necessary but not that he's a P M in Waiting. When Nixon resigned it was in a hurry so Ford had to step in, then he had to stand for election in the next term. It was the same when Kennedy was killed, Johnson had to take the reins then decided to stand in the next election. Tony Blair made it clear a long time ago that he would resign so there was no urgency for a replacement.