At LHR, which is the airport I think the OP was talking about, they absolutely check passports with the boarding passes when you join the queue. I’ve never seen them not do that, and I fly out of LHR on AC a lot. Not only does it filter out the people in the wrong queue, it makes boarding go faster as the ID is checked before boarding starts, and it prevents people holding up boarding trying to find their documents at the last minute.