Please refer to paragraph 16.3.1 of VAT Notice 700 ‘VAT Guide’, which lists the information which must be shown on a VAT invoice. There is no requirement to show a PO number and you cannot reject an invoice from a supplier which shows all the information stated in the Notice.
We cannot comment on any additional information you request from your suppliers. That is a commercial matter between you and them.
VAT guide (VAT Notice 700) 16. VAT invoices