What you've not told us is whether you're a UK citizen or whether you're in the UK on a Student ('Tier 4') Visa. If the latter applies then there are restrictions on the work that you can undertake. See here:
http://documents.manchester.ac.uk/display.aspx?DocID=31909
(Note, in particular, that you can't be self-employed which rules out quite a few activities).
Unless things have changed greatly since my own days as a student though, one of the most common ways for students to earn money is through working in pubs, clubs and bars or in other parts of the hospitality industry, such as in restaurants. Unlike, say, office work, the times when people are needed most to serve behind bars generally don't conflict with lecture hours. (With more and more shops, especially the big supermarkets, now open well into the evening, or even for 24 hours, there might also be jobs available within the retail sector at times convenient for students).