30 + years ago when I worked in checkin at LHR. the BAA provided wheelchairs and a wheelchair pusher. As it was usually easier to check-in the pax's own wheelchair rather than to load it at the gate, it was usual practice to use an airport chair to take the pax to the gate.
The BAA built STN so that you need to be a marathon runner to get to most gates. The BAA controls the airport, makes the rules and should, like they used to, provide wheelchairs for passengers that need them out of the vast profits they are making.
I belive that FR does a good job providing a reasonable service at a very low price. There is no way that they can afford to cover the £18 out of a £20 fare