I thought Aer Lingus tried that at Southend and it wasn't long before they scrapped the idea and the route. They had Southend to themselves and Heathrow is a slog from the Essex coast yet it failed.
It was a little more complicated than that, the route was operated by Stobart Air who foolishly signed a deal with Flybe in a desperate attempt to make Southend work, Flybe was handed Dublin and screwed it up. Aer Lingus Regional had decent loads in the end.
Doubt they would make it work from Luton though.