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).
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.