This gov link sort of fleshes out the process. Their starting position is "complain directly to the person or organisation; use someone else to help you sort it out (called ‘mediation’ or ‘alternative dispute resolution’); make a claim in a court or tribunal". So for example you can complain to the director of the library. This may result in the reply "we're sorry, we can't", so you seek mediation (presumably a university ombudsman). If that doesn't work in your favor (and if you still believe you can compel the university to create some alternative medium), then you take it to the courts. EASS is happy to help.
The court will then try to determine if you're suing the right person (it could be the publisher). Under the Equality Act 2010 Chapter 2 Section 20, there is a duty to make adjustments:
Where this Act imposes a duty to make reasonable adjustments on a
person, this section, sections 21 and 22 and the applicable Schedule
apply; and for those purposes, a person on whom the duty is imposed is
referred to as A
One of the central questions would be whether it is reasonable to require a library to create a particular alternative format of a given book. Let's say that you are taking a class on history of science and one of the required texts in Bacon's Novum Organon, and you require an audio version of the text. If you cannot comprehend written words but can comprehend spoken words, you are at a substantial disadvantage relative to others, so the university has a duty to eliminate that disadvantage. Then it is simply a matter of determining what actions could eliminate the disadvantage. (In this instance, it is trivial: direct the student to one of the free audio versions, but in some instances, it is an actually hard thing to do).
You should presume that your university is aware of the law, so you can inquire about their methods for complying with this act. If, by "challenge (the policy)" you mean that they have refused to make accommodations, then you probably need to initiate a legal challenge, whereas if you mean "how do I get what I want", you start by asking the director of the library how the library will comply with the act, given your specific circumstances.