Sorry to say ST but they do scan.
There was a considerable build up of arriving passengers at LHR T1 on Friday afternoon = perhaps 100 people queueing for the 3 UK/EU/EEA/Swiss Immigration desks. Normally I cruise through almost without stopping.
By contrast, the 'foreigners' queue was short.
It turns out that they are swiping all passports. To what purpose, I wonder. Hardly to maintain a log of arrivals and departures (not that that would be right for UK citizens anyway), when passports are not checked on departure.
Some travellers were questioned. The person in front of me was asked four, of which one was about his frequency of travel.
My own polite question 'why are you swiping passports?' was met with the non-answer 'yes, we're swiping all passports now'.
I was under the impression that UK citizens did not actually need a passport to enter the country. Now it seems a machinbe-readable passport is needed.