The Europeans are very precise in the way they think about rights. If there's a single market, there should be one set of rules. If there's one set of rules, it should apply equally to all citizens no matter whether they are disabled or their mobility is restricted in some way. Opportunities for travel should not be denied people because they are disabled. Except. . . There are always exceptions. In this case, it comes down to questions of practicality and safety, so we need to explain carefully how the rules are designed to work. Before an airline or travel agent accepts a booking from a person with a disability or some form of limit to their mobility, there's a duty to identify any problems likely to occur. This is not something that should arise when the disabled individual arrives at the airport. If proper notice is given in advance of potential problems, refusal to allow boarding will be considered a breach of contract and entitle a full refund plus the award of compensation. This applies whether this was full-price or cheap air tickets are involved, and whether this is flight only or part of a package holiday or tour.
This duty to accommodate the disabled does not just apply to the airline. It also applies to the airport operator which should provide properly designed access and trained staff. This should not be separately charged. Societies should be inclusive and not discriminate against individuals on the ground of their disability.
So when can transport be refused? In less common cases, because the disabled individual cannot pass through the aircraft door or something prevents moving through the aircraft to the designated seat. If this proves the case, the carrier must suggest alternatives. This can involve requiring a second individual accompany the disabled person to offer help and support. If a person with limited sight has an assistance dog, all airlines must publish their rules and cannot ignore them when convenient. If the disabled person holds cheap air tickets that involve changing flights, each airport has the legal duty to ensure appropriate arrangements are made to move from one gate to another.