I also think it has to do with the Rate Matrix only. I am assuming that the exchange rate is filled up correctly for converting GBP to USD. But still, you should check there once as well.
So, basically, two places to check, Rate Matrix to find out if the RATES are defined for that resource and then the Exchange Rates. Need to make sure that the exchange rate is also defined between GBP and USD, and that needs cover the transaction time period. Also the conversion rate should not be Zero (0).