Soapbox rant -
what would Aesop do?
Make a condition of licence to commit to low-carbon travel.
The company with the best low-carbon score gets the slot.
How clear your commitments are will determine your eligibility for a slot.
You get a licence for London to Barcelona if you can demonstrate you are supporting an equivalent journey by a lower carbon means.
It need not be the same route, but that might score it higher.
Would shift the incentive to airlines to provide the solutions.
Run a night train from London to Barcelona. Better still, start it in Manchester and let it run to Malaga.
Run a train for holiday makers to a different destination.