There is a 777 even operating into Luton. So it's possible. And major infrastructure expansion is happening at LTN over the next 10 years to accommodate double capacity. EZY large base which taps into the skywards deal.
No premium carriers from LTN or STN so personally i see the option to access EK's route network via Dubai as a great opportunity for those north/ne London.
They are both dumps but so is Newcastle, Birmingham, Glasgow etc....and with easy access to london has a large premium market on its doorstep ontop of the usual bucket and spade types.
LHR and LGW are high loads, limited ability to expand due to slot constraints. So extra seats targeting this sector may not affect the current loads they are seeing. STN needs something to save it, I'm sure they could offer a good deal.
Anyway sure they have looked into it.