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.