It is very important that you are aware of the restrictions on working in the UK and never work when you are not allowed to do so. Opportunities to work after your studies are limited so start finding out about your options early.
If your employer has asked you to get permission from Brookes so you can work, you need to direct them to the Home Office as we cannot do this - see Preventing Illegal Working: guidance for employers (page 51). There is also a UKVI Employers’ Helpline they can ring: 0300 123 4699 or 0300 123 5434.
What you should provide your employer with is confirmation of your semester dates - see How many hours can I work.