If you hold a Tier 4 visa, it may be possible to undertake a period of 'work placement', during which time you can work full-time.
This is only applicable if your academic department considers this to be an 'integral and assessed' part of your course (meaning you are enrolled on a module which is credit bearing and the work placement forms part of the assessment), and the total duration of the work placement is no more than 50% of the duration of your degree course.
The only exception is for courses where there is a statutory requirement for more than 50% of the work placement (this usually includes professional courses such as PGCE and MBChB).
If you are unsure, you should approach your academic department for further guidance in the first instance.
There is no prohibition on receiving pay during the work placement.
Any work placement you do whilst on a Tier 4 visa has to be an integral and assessed part of your course. If not, then you will not be able to take a placement full-time during term time.
If you are an undergraduate student, you can take an internship during the university vacations as this will fit in with the working conditions linked to your Tier 4 student visa, which state that you can work up to 20 hours a week during term time and full-time during vacations.
If you are a postgraduate student, you do not follow University term dates and can work/do an internship up to 20 hours per week during your course registration period. The only exception is after the official course end date, when you can work full-time provided that it is on a temporary or rolling contract and not a permanent position.