It would be pretty easy to shift all internal flights from Heathrow to Northolt . No requirement for custom or border control, could easily add Ireland as well as again no border control rules . In addition they have a nice new terminal in place already that could easily be extended.

Add that to relatively small planes on the effected routes and it's a quick win to free up substantial capacity at Heathrow for relatively modest cost. It's hard to see the business case not stacking up.