As a Student Union Sabbatical Officer you are expected to take leave from your studies for the purpose of undertaking a full-time, salaried, elected, executive union position. You therefore are not permitted to study and work full-time.
If you want to continue with your studies whilst you hold a Sabbatical Officer position, you should speak to the Guild before you apply to become a Sabbatical Officer to discuss the expectation of the role.
The Student visa only allows you to undertake ‘supplementary study’ such as evening classes or short courses which does not contribute towards your main course of study for which you were granted a Student visa.