The Europeans always try to produce an inclusive society. They don't always succeed, but there are an impressive series of laws available to reduce the risk of discriminations. Worse, not all states are equal in the way the laws are interpreted or enforced, but these opening steps have been taken to produce a practical equality of opportunity at some time in the future. Without them, discrimination would be significantly worse. One of the affected groups are the disabled or those who find they cannot move around freely. For the record, this does not include the obese. For better or worse, they choose whether to restrict their mobility.
Disability as a class only includes those who find themselves unable to reach average standards of ability due to an accident of the genes, disease or injury. So, if a seriously overweight person cannot physically sit in a seat provided by the airline, that's not the airlines fault. The choice is either not to travel or to buy two adjacent seats. Put another way, if airlines have to refit their airplanes with wider seats, this means fewer paying passengers to all must pay higher ticket prices to cover the same operating costs.
But the physically disabled cannot be victimized. So long as they can physically get through the door of the airplane, they can sit comfortably in the seat provided and there are no safety issues caused by their presence, the airlines must honor the tickets and the airports must be adapted to make access easy whether the tickets are full-price or cheap flights. This means that before the booking is made the intending passenger is required to clearly identify their needs.
The travel agent or the airline must either accommodate the passenger or make suggestions that are practical as to how travel can be made without making prices unfair once they are aware of the potential problems. This includes permitting guide dogs for those with visual disability. If the airline fails to allow boarding once the passenger has arrived at the airport for checking in, not only must there be a full refund but statutory compensation will be payable and there can separate civil action and government enforcement by way of fines. Cheap flights must be equally available to all citizens regardless of ability.