Ignore the HUD,
How about the following suggestion.....
Legacy airlines have a very mixed fleet and there may be tech reasons for one to be substituted for another. The 24 hour restriction enables them to swap toys round without having the hassle of dealing with passengers moaning about seat-swaps.
Airlines such as Ryanair and Easyjet have a more homogeneous fleet ... an A320 is an A320 ..... so you can choose your seat in advance and it doesn't matter what A320 you end up on.
With BA, you might for example book your flight on something that was originally scheduled to be a A320, but when that goes tech they might brush the dust off an old 737. Or BA might swap one 747 layout for another 747 layout.