This does not happen consistently. For example, if travelling hand luggage only on British Airways/Aer Lingus, your passport number is not checked at all – it does not appear on the boarding pass, and staff at the gate do not look at anything other than your boarding pass and passport.
At Eurostar, a Eurostar staff member swipes your passport before you leave the UK terminals, so it is likely this journey will be recorded accurately. I have not taken a ferry in some time So cannot comment on that.
When flying outside the CTA, if you wish to check in online you must submit your passport number to the airline. If travelling hand baggage only, then yes, occasionally your physical passport details will not be validated by a human on that particular trip.
If you entered a false British passport number I am close to certain that even if you managed to get a boarding pass, it would be rejected by the time you came to board the flight; also BA definitely submits the API to countries like the USA, Canada and Australia and if an invalid passport number was entered online check in would fail.
If your physical passport is not validated on that particular trip, it will have been seen by BA at some point in the past. I don't know about Aer Lingus as I have never flown with them.
From my and others' experiences with BA flying around Europe and the world pre-Brexit and pre-COVID, after around 10 trips HBO, I would be unable to issue a boarding pass online and I would have to swipe my passport at a self-check in machine at the airport, occasionally this would also fail and a human would need to see my passport. The only reason that it didn't happen as often as it might was probably that I usually check baggage on long hauls which does require a human passport check, which may have "reset" the counter for being allowed to fly with only a cursory check of the name on my passport