Section 23 of the Disability Discrimination Act makes it unlawful to discriminate on the grounds of disability in providing access to or use of premises that the public can enter or use.
The purpose of this Act really to encourage us all to provide access, goods and services in such a way that disabled people can be full members of our community. It's clearly not always about making physical changes to buildings to do this (eg It's not possible to reduce the wingspan of planes to allow the gates to be moved closer together!).
It such circumstances the only practical solution is to provide additional support services and equipment for the disabled. To charge for these services directly is against the spirit, if not the letter of the Act... and so it jolly well should be.
So who should pay? According to discussion on TV (Ok not the best source) Ryanair had to pay because airlines are responsible for getting pax from the check-in to the gate. Apparently BAA is only responsible for getting pax to the check-in. That sounds strange to me but if that is their contractual arrangement with BAA then it would seem that they are obliged to pay up.
The only remaining issue is the impact on ticket price.. Ryanair say it's 50p, but, according to the TV, a dissabled group calculated it at 2p. Eitherway it's not even small beer.
Overall I think Ryanair have handled this case very badly, what were they thinking. 50p may be significant to them but not to pax. Many Ryanair flight are practically free - it's the tax that's the problem.