There is a Multi-Currency option that once implemented applies to both AR and AP. You would set USD as your Base Currency and British Pounds as a source currency. All customer and vendors would be set to USD by default and then the British vendor would be set as Pounds. You need to set up the conversions so you can recognize the gain/loss on exchange between the conversion at the time of PO receipt and payment processing. See IM-A International Configuration and IM-B Enter Multiple Currencies