Add this "product" to the shopping basket to reduce the amount to pay.
Change the quantity to the number of UK pounds (GBP) to be credited and optionally add a short description in the Credit For field below the price. Click the Add to Basket button and then check out normally.
When exporting outside the EU VAT is deducted, so this actually credits only 83p not £1.
Do not add this product to your shopping basket unless you have been explicitly authorised to do so.