I speculated a few weeks back, that now Vueling and Iberia Regional were associated with Luton, it would seem logical for Aer Lingus to operate to link into their growing long-haul and pre-border clearance from Luton?
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.
At Luton they have Ryanair who could up their service at the drop of a hat and with Heathrow just a few miles away I could not see demand being that great for connecting flights because that is the only business they would have.