Yes, I have flown internationally to the UK with hand luggage only and the single time my passport was actually scanned was at the British immigration facility in the UK itself. At the gate, my passport was visually checked to see if the name matches the name on the boarding pass, and probably also to check that I have my passport with me, but it was not scanned.

APIS information was requested, BA offers a form for APIS data in "Manage My Booking" as soon as your reservation is ticketed. If you don't fill it in before check-in, it will be mandatory to fill it in at OLCI, and IIRC the APIS form always appears at OLCI and will show the data you filled in before if you did, allowing last-minute changes. If you still don't fill it in at OLCI, you won't be able to finish check-in/get a boarding pass and will need to see a check-in agent at the airport, who will presumably scan your passport so he/she doesn't have to type all the data, or scan your passport at a check-in kiosk.