Maybe there is a question of interpretation here. I have flown out of LHR at least 20 times this year and obviously back again from a wide range of places. On all occasions I've had my boarding card checked at either the point of entry to the lounge or on exiting the lounge to the jetty, depending on where the machine for processing them is. On none of the occasions have I been asked by cabin crew if they could see it as I boarded the plane. Flights so far this year have been with eight different airlines and in the next month I'll add another two.
The reason why I asked whether it was company policy was because it's obvious that a lot of the crew posts are from BA staff and I never fly BA.