You're not wrong. It is 24 hours.

However Plus fares are changeable on the day of departure, up to one hour before departure, and can be done via the App. That's a free change made on the day of departure, subject to seating availability in your cabin. Basic fares, formerly know as Hand Baggage Only fares, can also be changed online within an hour of departure but that has a £60 change fee and fare difference.

Now I'm not sure this works very well in your case, but what I would normally suggest is that you book a Plus fare for as late as possible, then use the App to pull forward to an earlier departure on the day, if circumstances allow.