Not sure how Northolt would work in practice, it's not taxiabale to the central area so there's no actual connectivity. I mean someone landing at Northolt connecting at LHR would surely find it quicker at AMS or CDG and FRA. It's a competitive disadvantage. Connection times would be much higher.
There's enough moans about coming all the way in from T5C to the UK Border, a five mile train journey is not something you could sell in the market.