You are allowed to work up to 20 hours a week while on a student visa. However, in order to be issued a student visa, you have to show you can support yourself fully (fees and living costs) without having to work.
International tuition is around £10,000-£15,000 per year (depending on course and university) and living costs are estimated at between £7,000 and £10,000 per person per year (depending on city - London, for example would be at the higher end of both figures), so you would need to prove to the consulate that you had at least £17,000 per year or so available to you before you enter the UK.
However, once your visa has been granted and you are in the UK, you are allowed to get a job and work up to 20 hours per week during the school year - it just can't be your main source of funding while living here.