In your current situation, you cannot extend your visa as immigration sponsorship is being withdrawn and you are an external student.
Next steps:
The University is obliged to ensure that you are aware of your options. In this case, please note the following:
University of Birmingham and your student status
The current position is that your immigration sponsorship is being withdrawn and you must not stay in the UK beyond the visa expiry date, and this may have an impact on your ability to apply for a Graduate Route visa.