Many thanks for all the feedback.
CX Diamond has reminded me of something I’d forgotten.
Years ago LH used to operate its A300-600s between FRA and LHR and these had fixed business class cabins.
These A300-600 flights operated into LHR two or three times a day. They were timed to link with the waves of long-haul flights in and out of FRA.
Interestingly, LH used to allow a long turnaround time in LHR (two or three hours, if I remember correctly) which I imagine was to allow for recovery time in case of delays etc.