I was about to post this yesterday too! But I thought someone MUST have posted it before and I'd just missed it.

I booked a flight to LHR, the late evening from IAH, early April, and it's wide open, according to the seat map (which I know isn't reliable)..... I was surprised.

I would think that some people/uninformed TAs who booked into/out of LGW might not have known about the LHR option, so as knowledge of that option becomes more widespread some will "wish I would have known that", and now they can rebook to LHR.