EU law requires an airport to provide mobility assistance to those requesting it. The law requires the cost of providing such service be spread across all users of the airport and is included in the airport tax/passenger service charge paid by the passengers of all airlines that use the airport.

I will add the the special assistance that my family has received over the years at LHR is by far the worst service. It is dreadful. I have complained so many times I’ve lost count, nothing changes apart from it getting worse.