Hmm. The Disability Discrimination Act says that people can't receive less favourable treatment regardless of disabilities. This has an impact across the whole country - from access to public buildings to job recruitment. There is no reason that what happens at airports should be different.
Who should pay? Who said the government? In case you don't know, money that "the government" pays comes from the taxpayer - so you are effectively asking not only every other passenger to pay, but everybody in the country. Seems pretty arbitrary, if you never use airports ....
I would plump for the airport. However, STN is run by BAA - which still behaves like a branch of government - it fails to treat the airlines as its customers (which they are - i.e. the people who they are paid by); it feels happy to get airlines who aren't going to benefit from (e.g.) Terminal 5 at LHR to pay for it - up front - and basically any "business risk" is passed on to the airlines. Oh, and the government backs them up in this regard. So fat chance of that, then.
This being the case, it is left to the airline. The real issue is not that disabled people have a right to equal treatment, it is over where the liability for that equal treatment ought to lie.