Student Unions and Student Associations exist in UK
universites and
colleges, in the eyes of many students, as drinking establishments.
They do, however, provide other services. For example, cheap shops, subsidised canteens, a laundrette, financial support for clubs and societies, and upholding the interests of the students. Oh, and they have cheap beer.
Some unions also have concert halls, and many bands have toured student places before moving on to larger venues.
You'll often find a travel agent (like STA Travel or USIT), and an insurance firm (Endsleigh). And most definetly more than one bar, and a club.
The student union is generally somewhere people head en masse following an exam, the last lecture of term, or, well, instead of a lecture for a cheap drink on campus. And just about everybody gets chucked out of it at some point in their time at uni.
Some universities have a choice of union - for example, Glasgow has the Glasgow University Union and Queen Margaret Union providing shops, bars, food and clubs, societies, and venues. The Students Representative Council has shops, food, student support, and student publications. Everyone is covered by the SRC, and students can then join the GU or QM.
Many student unions are affilliated with the National Union of Students