Food (not catering) is usually zero-rated and should be entered on CUFS using the tax code 'No UK VAT', but there are a few exceptions on which you will be charged VAT at the standard rate - see section Sales/Income in the UK, Food [1] for standard rated food items.
Links
[1] https://www.finance.admin.cam.ac.uk/policy-and-procedures/financial-procedures/chapter-9-vat-and-other-taxes/salesincome-uk/food