It is easy to invoice a client that is set to a currency other then your base currency. This is often called charging in foreign currency. Simply make sure that the client you wish to invoice has the correct currency set. Then create an invoice in a heartbeat by pressing Create new button and select Invoice.
Then you will get the outstanding balance view.
Important: In this view all amounts are converted to your base currency to help you realize that outstanding balance better. When you press any of these matters to issue an invoice you will start working with the client's currency.
Then you just proceed with billing. Simple as it can be!
How can I set special prices in different currencies?
But if you wish to define a special price in GBP that applies as a new list price in that currency. Then check out this article and read the chapter on Prices in different currencies. It is very easy to do!