If you use your card to make a purchase in a different currency than EUR, GBP, USD, the purchase will be converted to the currency your card was issued in (GBP for UK users and EUR for EEA users) at the exchange rate shown on the VISA website. The rate is calculated on the date when VISA authorises the transaction.
In this case, the exchange rate is not fixed by us, so we cannot be held responsible for providing a good rate. Exchange rates can change at any time, without warning. You can compare the rate applied to your transaction with those published by the European Central Bank on this link.