LHR/MAN ,unfortunately ,has not been one of BA's most profitable routes , certainly not in the last 25 years , if ever. ( even looking at the shorthaul side let alone the network ). Its raison d'etre , has also been to feed traffic connecting on to the rest of the network .Not only did it have one of the highest % of connecting traffic on domestic and shorthaul as a whole but a fair part of these connecters where high yield Premium psgrs
In its heyday in 80s/90s there was also a lot of pt to pt necessitating 767s on peak rotations but this was killed off by a combination of shorter rail journeys and the increased checkin times and processing times on such a short sector post 9/11.Therefore the route has shrank back to mainly connecting.
A lot of BMI traffic is also connecting , most likely to *A partners and the demise of a BMI operated service will probably see this traffic migrate via other *A gateways rather then to BA.I don-t think BA will alter freq/capacity significantly unless they have to politically.
EDI is different as you suggest.
It has consistently been the top domestic performer with a high proportion of high yield pt to pt though connecters area also significant.
BA would need to increase frequency / capacity on this route in the event there is no longer competition on this route in order to satisfy demand.
I guess this is a bit off thread (BMI).