Under the law passed during the last parliament, the date that the next election must be legally held by is 25 working days (35 ordinary days), five years after the formal meeting of the Parliament. This Parliament will formally meet on Wednesday for the King's Speech (July 17th 2024), therefore the last date for the election will be August 21st 2029 (a Tuesday), therefore the last practical date will be August 16th 2029 (a Thursday)

Yeah, that’s the reason I didn’t include a 1/1/2030 option as barring something insane it should default to 100%. TBH I suspect the actual percentages should be closer to 50% / 50% on the 2029 option but four years is a long time in politics.