A Student route visa holder can do voluntary work if they are permitted to work, but this work and any paid work must not exceed the total number of hours you are permitted to work during term time. For example, if you are permitted to work 20 hours a week during term-time and you have paid work of 15 hours a week, you cannot do more than 5 hours voluntary work in the same week.
University’s working restrictions
The University can impose tighter restrictions on the number of hours you can work and you are expected to abide by the University regulations. It is recommended that you check if your School has any particular restrictions on its students. You should adhere to the University restrictions on working during your studies.