UK universities have a statutory obligation to support their students in the establishment of some form of Students' Union (sometimes also called a "Students' Association" or "Guild of Students", and, in the Scottish Ancients, a Students' Representative Council).
Whether or not universities actually do conform to such statutory obligations, and if, for example, the code of practice of the NUS is followed when determining the make-up of such bodies is a hotly contested and ambiguous matter.