Sorry what? BA didn't walk away because they didn't like the North, they were at a competitive disadvantage. On MAN-AMS say, KLM have a mix of point to point and spoke to hub. EZY entering this market can then pick up the bottom end of the leisure market leaving BA with expensively priced point to point only but without the cost base to match EZY's, and requiring the capital investment to go against a horde of new Airbus narrow bodies. Of course EZY didn't stop at the leisure end and now have good reputation amongst business travellers.
Can anyone here honestly see BA flying MAN-AMS on an A319 and making money? Because against EZY and KLM, an EMB145 just won't cut it. They'd be loser meat in someone elses sandwich. One thing IAG are rightly ruthless on, is that it must make money or go away. BA are competing against IB and more worryingly VY for the new A320 NEOS IAG are taking, more cost cutting is needed to make LHR short haul into the black at last. Not one person on here has ever shown me a persuasive case as to what a profitable BA operation at MAN would look like! Lufty are even retreating from regional Germany and "rebranding" as German Wings in a bid to save EZY stealing market share. It's not going well either! Also remember BA at MAN/BHX was a hub fed from Scotland, nowadays, Scots have many more cheaper and direct options with the locos.
There never was the remotest business case for BA putting in the rather high capital investment in new A319s to make MAN workable in a cut throat market against the locos. The return on investment would have been massively negative!
There's no point flying loads of passengers out of regional airports to drive volumes up if you cannot do so at an acceptable margin.