geeta,
The currency in the billing document would be same as that in the sales order.
If user is not ready to reverse the cycle, then I think having the currency in the billing document as GBPN would not be possible!
Additional comment: The currency from the customer master record flows to the sales order and then the it flows from the sales order to the billing document.
If the customer master has a different currency than that in the sales order (due to user changing the currency in the sales order manually), then the currency in the billing document is same as that in the sales order (and different from that in the CMR).