I absolutely know the answer to this as I've sold in a product category that contained a mix of zero and standard rated goods, and the vat charged on the out-going p&p follows the flow of vat charged on the items being dispatched. If you charge £5 p&p and the order contains items that have all attracted standard rate 20% VAT, then the £5 P&P charge is actually £4.17 p&p and £0.83 VAT. If you charge £5, and the order contains items that are all zero-rated, then the £5 charge is £5 p&p and £0 VAT. If it contains a mixture, then the p&p will follow the weighted VAT rate of the goods in the parcel.
Am I now in line for most nerdy post 2013?