Tarkus1212
Challenge Tour Pro
You are supposed to use both bits according to UKB, the photo Id bit is quite easy to forge, but the paper bit isn't so if they are sus of you and you were using that. they ask for the paper bit as back up.
Like others I have often used my driving licence as ID for domestic flights (Flybe & Easyjet) and have never been asked for the paper part. My ID is only ever checked by the airline staff as there's no need to pass through Border Control. On flights to other EU countries foreign nationals seem to be able to travel on their national ID cards without any issues when passing through UK Border Control.