fixed price issue on checkout

Hello

I have setup a product with 2 currency like first US dollor and second is GBP britis dollor. I have enter second currency price fixed into product. my default currency is US dollor. once i have added product into cart with GBP price then price show correctly into cart but on checkout page product price which is GBP its show currenct conversion price.
example :
product : http://shopbooksdirect.com/product/fall-in-love-with-rye-2
price: US65.00/- and GBP40.00/- (GBP is fixed price which set into product).
select currecny : GBP
then add to cart
but I go to checkout page then price is showing £51.16 which current conversion rate.