Happy to help here - It sounds like you may need to edit your Google Checkout Merchant Account settings. Your merchant account is denominated in your country's currency. (For example, transactions are denominated in USD for U.S. merchants, and in GBP for U.K. merchants.) However, the default currency for Google Checkout transactions is USD.
Have a quick look at your Checkout account and confirm if it is set to GBP. If it is, then let me know and I can file a bug with our Eng Team.