Worth remembering that no Scottish city is within 2 to 3 hours surface journey of London, so wouldn’t you kind of expect there to be greater demand for domestic air travel between Scotland and LHR, than there would be to parts of Northern England?
Also, Scotland’s airports aren’t nearly as well served for direct international flights as MAN, even though things are slowly improving, so again the LHR flights remain important for connecting Scotland to the wider world.
Edinburgh is the second most visited city in the UK, and Glasgow and the Highlands also rank quite highly. Also, factor in Aberdeen’s exceptionally strong demand for business flights, and it’s not hard to see why BA might determine that there is a greater call for flights to the Scottish airports than there might be to airports in northern England. Simply looking at population factors close to each airport doesn’t tell the full story in terms of total demand to/from an airport. You have to look beyond the UK to see what the inbound demand is like too!