You can buy up to 5 different currencies in each instant order, as long as the value of your basket does not exceed £10,000 (or your remaining annual allowance).
If you only require one currency, simply specify the quantity required and chose the "checkout" option.
If you require another currency, you can either return to the basket from the checkout by choosing the "back" option in the basket, or if you are still in the basket you can simply "add" another currency by selecting it from the drop-down and specifying the quantity required.
You can "edit" the value of each currency while in the basket for an updated value, or "remove" the currency altogether.
If you have started the checkout and locked in the exchange rates, you can still change the quantity or remove a currency from your order without returning to the basket and losing your locked-in exchange rates.