This was something that was offered us at our hotel when we checked out.

We didn't take it as we were flying back club and I didn't think it was worth it. My understanding is that there is a dedicated check-in and security channel in a separate part of the terminal, which takes you straight into the lounge used by BA. My guess is this is a good option if travelling WT or WT+ but doesn't give you much benefit if in CW or F. Check in for CW was a breeze, but we were there the 'required' 3 hours before departure at the insistence of our transfer operator!