To address your initial question "Which has more C availability - LHR/ORD or LGW/DFW?" - I have wondered this myself. I have actually never been waitlisted LHR-ORD, but have been waitlisted every time LGW-DFW. The time of the request (weeks, months) seems irrelevant. (I am still waitlisted for an LGW-DFW upgrade a few weeks from now, have been for months and the cabin is almost empty.) It must be the actual number of available flights per day that causes the squeeze - needless to say LHR-ORD wins.