WITHIN 72hrs. So 3 days before or 2 days before or the day before. Mad to leave til the day before though. Account for website issues, delays, unexplained weirdness 🤷♂️
UK has visa exemption for 60 days. You will have to fill out the new online arrival card 3 days prior to arrival though (at least 3 days before arrival, but access isn't available until the 28th of April i think)
I'm pretty sure it's 3 days before your intended arrival. It might be causing problems because it hasn't been officially implemented yet. I guess after