VAT code on Purchase Order from Supplier and not the Stock code: Merged

Customer has some products that are non VAT registered but when they sell the fitting charge, it is standard rated for them. They need the Purchase Order to pick up the VAT status from the Supplier detail and not the Stock Code. So I.E - GB suppliers pick the stock item VAT code whereas the non GB Suppliers use the Supplier account VAT codes, the customer would like a change request at the point of order entry instead.