Earlier quoted context omitted.
The UK doesn't have exit checks.
Of the ~50 countries I've been to in the past 10 years, The UK is the only one I remember which I did not need to show any ID when leaving. I thought it was strange because this is just one way countries catch overstays.
Exit checks are pretty pointless if there’s no violation that would lead to enforcement other than deportation (since the traveler is already self-deporting).