If you are a student from outside the EEA (European Economic Area) and Switzerland, and have permission to enter the UK for a period of full-time study lasting more than six months, you are usually able to undertake employment in the UK without having to apply for additional approval. This employment must be on a part-time basis during term-time (up to 20 hours) but can be full-time during vacation periods.
If you are a student from outside the EEA and Switzerland and are coming to the UK for less than six months, you may work on the same terms provided you have secured a Tier 4 student visa. If you have obtained a short-term study visa you will not be permitted to work in the UK.
Please note that these restrictions on 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.