On the conform page, in the code that passes the transaction to the gateway, there will be a variable that sets the currency.
for example, using Paypal Pro, there wouuld be the following on the confirm page:
$PP_DirectPayment_required[0][$nextIndex] = "Currency";
$PP_DirectPayment_required[1][$nextIndex] = "USD";
change that to:
$PP_DirectPayment_required[0][$nextIndex] = "Currency";
$PP_DirectPayment_required[1][$nextIndex] = "GBP";
to use Pounds on paypal.