If you are a student from outside the European Economic Area (EEA) and Switzerland and have permission to enter the UK for full-time study for a period lasting more than six months, you are normally able to undertake employment in the UK without having to apply for additional approval. Such employment must be on a part-time basis during term-time (up to a maximum of 20 hours) but can be full-time during vacation periods.
If you are coming to the UK for less than six months, you may also work on these terms providing you have secured a full student visa. Please note that these restrictions on undertaking employment apply both to paid work and voluntary work. If you are an EEA or Swiss national you will not be subject to these restrictions.
Please see also the Immigration advice on this page; changes to visa regulations may impact on employment regulations for affiliate students.