in order to do this you'll need to configure multiple websites. Magento has one base currency per website, which means that all other currencies within that website are FX conversions from the original one.
So, for your example, if the base currency was USD then GBP would convert based on a rate from that original currency.
You can do some more reading here: http://www.magentocommerce.com/knowledge-base/entry/managing-currency/
hope that helps
