" The main purpose of the Tier 4 visa route is for you to come to the UK to study. Where you are permitted to work, the intention is that this should be to supplement your income whilst studying (which in most cases we expect this to be taking a part-time role for an established business/organisation)..or as part of a work placement provided it is an intergral and assessed part of your course."
Under the UK immigration rules, you are legally permitted to work with the following restrictions:
a maximum of 20 hours paid or unpaid work per week during term time for degree students
a maximum of 10 hours paid or unpaid work per week during term time for Language Centre students
full time work is permitted during vacations
you cannot take a permanent contract (open ended) until you have submitted an application for a work permit e.g. Tier 2
no self-employment (including freelance or consultancy work or setting up your own business, even if you are not the controlling shareholder)
no work as a professional sports person (including sports coach) or entertainer.