There has consistently been very high demand in the premium cabins on the LHR - MIA route since late last year, with very limited availability.

From what I can see cruise passengers account for a large proportion of the pax (MIA covers cruises starting both in MIA and SJU), and I was told that the cruise operators block book vast chunks of seats, which they then give back to the airlines if not required (although I have never been in F or J when they were not rammed full).

There could also be the possibility that some premium cabin seats are strategically withdrawn, to accommodate overselling of WT/WT+, although I have no idea if this is something that BA does ??