In the interim period between the previous monarch dying and the new one canvassing for votes, how would legislation get approved? How would elections get called? How would the Prime Minister be appointed? Parliament opened? How would all the things the monarch does as part of our constitution happen?
The reason that succession is immediate is because the country can't run without a Head of State. Plus of course as a PP pointed out voting costs money, a lot of money. I don't think this idea is practical. You either have a monarchy or you don't, this is a sort of constitutional half in half out and we know that doesn't work.