If you are using paypal; Payments Standard, look for the following code:
$PP_DirectPayment_required[0][$nextIndex] = "Currency";
$PP_DirectPayment_required[1][$nextIndex] = "USD";
and change it to:
$PP_DirectPayment_required[0][$nextIndex] = "Currency";
$PP_DirectPayment_required[1][$nextIndex] = "GBP";