Because it is normally politicians, ex. Leaders of parties, ex ministers who get nominated for these Head of State largely ceremonial positions. I suppose they could be seIf proposed, if there are two or more nominees it goes to a national election.
Well before then there will have been a referendum to establish a Republic and propose the dissolution of the monarchy. Parliament will decide the terms of reference for a head of state. What powers he or she has, if any. It will then go to a commission of the great and good to review all acts and statutes in law, determine who owns what in terms of the duchies etc.
Almost like a certain withdrawal act in fact.
Of course we could go all French and chop off their heads. Angela Rayner and Harriet Harman sat knitting at the foot of the scaffold. Watching the heads roll.