BA won't be adding any routes (domestic or otherwise) that are not "thick" enough for at least the A319, it does not have any smaller aircraft. That much is obvious.
BA runs the domestic London City to Isle of Man with a chartered Saab 2000 in full BA livery. All is possible where it is desired.
Given Heathrows cost base, just how does anyone intend to make a J31 into Heathrow profitable?
Heathrow's charging is a regulated pricing, and the government can set this, provided it is not found unreasonable, as they wish, so the political desire for a proportion of the new runway capacity to be used for regional services can be met in this manner. It has not been hitherto possible because the EU rules that there can't be any pricing or rules for domestic-only operations, any such rules had to be for Europe-wide services. This obviously no longer applies.