The A321XLR might open up opportunities in the distant future for regional airports, be it on BA or other airlines but BA would probably say passengers have a wider choice of their flights and OneWorld carriers by travelling via LHR and connecting.

I also think BA crewing agreements do not lend themselves to regional bases - even London cabin crews are either LHR or LGW based rather than being available to operate from either. If anything, Manchester would probably be on the BA longhaul network in future but many will probably argue that the regional flight profile is more leisure heavy traffic. But yes, 3 or 4 weekly 788s to the East Coast or even DXB, SIN and others would probably be full from Manchester.