Do not forget the ferries, and the goods in transit.
There was a reason I asked this earlier
Just out of interest, do you have any experience of being a European non-EU/EEA citizen dealing with the UK immigration system?
Or indeed being a UK national dealing with a European non-EU/EEA country and their immigration system?
When my partner first came to the UK, her country wasn't in the EU/EEA yet.
She had to jump through all sorts of hoops to get a study visa.
(And me vice versa to get a work visa to her country).
We decided to take my car to France for a short break, and it almost ended in disaster.
On our return, at the French end they questioned why she hadn't got a stamp in her passport when leaving the UK. I hadn't thought about this as we (UK) had 'free movement'.
All sorts of discussions went on and one option was to refuse boarding and return her to her 'country of origin', which was her own country due to no stamp in her passport when leaving the UK!
Luckily there was a human in the border post, and what with my poor attempt at French we got the following ferry, as by that time the original one had sailed.
When we got to the UK we had the same problem - you can't come in because we have no proof you left. Again there was a human behind the uniform, and she was let in. However they must have alerted immigration, as a week later we had to explain it to an official who turned up at her place of study.
They asked if we planned to go to abroad again during her study visa period, and we had a flight booked the next month. The advice we were given was to always get the passport stamped on exit. Fair enough. The hassle we had getting that stamp at the airport was unbelievable - "we don't do stamps for European countries anymore"...
Wtf...they didn't even know the difference at 'border control'!
And this was many years before a 'hostile environment for immigrants' was thought of.
After a couple of decades of not worrying about this again, I guess it'll soon be back to the 'good old days' when travelling between our countries!
Edit: That was just a minor problem when compared to getting the full 'indefinite leave to remain' - which is apparently about to become invalid!