The tax code for entering all purchases charged without VAT (within the UK) on CUFS is 'No UK VAT'. This is irrespective of whether the supply is zero-rated, exempt or outside the scope.
This does not cover medically exempt purchases which should always be entered on CUFS using the tax code ME0% (please see Medically Exempt purchases [1]).
For purchases from outside the UK you may have to apply a self-charge VAT. The accounting mechanism and tax codes to use are covered in these web pages.
Links
[1] https://www.finance.admin.cam.ac.uk/policy-and-procedures/financial-procedures/chapter-9-vat-and-other-taxes/typical-purchases-uk-4