No-notice, turn-up-and-go international travel out of the UK will not, ever, be banned.
[Passenger] data are only mandatory when they are requested to be provided at the time when passengers are on board in preparation for departure and it is no longer possible for further passengers or crew to join the service. When it is requested before that time it only needs to be provided to the extent to which it is known to the carrier.
In other words, the 24-hour rule for data communication only applies to data that the carrier happens to have, and there is no requirement at all to collect data right up until the point where everyone has boarded the plane, train or boat in question and it’s about to set off. The e-Borders programme will do absolutely nothing to restrict people’s freedom to travel – it just means that information on where they’ve been will be passed onto the government afterwards.