I believe it has to be a BA-only itinerary, i.e. no linked non-BA flights. But you might as well call to ask. If there is an availability discrepancy, there's a chance they will see the wider opening.

But I'm curious where you are seeing the 7 seats. Only the 29th seems to have 2 CW seats BOS-LHR, and neither ba.com or aa.com come up with a routing from MIA to include that flight.

Is that the date you are looking for? There are 4 seats from IAH-LHR.

EDIT: Also some routings via YUL that might work but won't come up automatically.