You can use your card anywhere that Mastercard is accepted.
If you want to use your card for a transaction in a currency other than the 15 currencies you can load onto the card, just make sure that you have enough in the other currencies on your card to make the transactions.
We'd recommend you simply load enough onto your Sterling balance to cover any transactions you want to make and the card will debit money out of that balance to complete the transaction. This will happen automatically. You don't need to let us know if advance or change any settings, your card will simply pay out in the currency the merchant is charging you in, easy!
There's a 1.75% out of currency fee for transactions that happen in currencies other than the 15 available on your card. The exchange rate that would be used is the rate set by Mastercard for the currency you're paying in, which you can find here: https://www.mastercard.us/en-us/personal/get-support/convert-currency.html