You will be quoted all prices in Czech koruna and therefore if you choose to convert your Sterling into euro and then the euro into koruna, you will lose on the conversion twice - better to either buy koruna with your pounds or else simply use plastic on a Uk account and let the bank/whatever do the conversion. watch out for the rates, fees, etc. levied because some are better to use abroad than others. However, if you already have euro money as a base currency then using it to convert into koruna will not be any worse than for those who have Sterling as a base currency using pounds to convert into koruna.