New at LGW. In Departures at the South Terminal, after you have scanned your boarding pass, had your photo taken and passed through the barriers, you have to hand your boarding pass to the conveyor belt people as well.
Unfortunately, you don't learn about this while you are on the queue waiting your turn. By the time they hear about it some people have already put their passes back in their hand-baggage while queueing and now have to go rummaging in the trays thus slowing down the queue even more.