My sister is trying to buy a one way flight from Katowice to Luton, using Wizzair. The total cost is 308 zlotys. When she changes it to GBP it comes out as about £57. The official conversion rate would make it about £53. The airline is therefore charging about £4 for the conversion. Would it work out cheaper for her to buy it in zlotys and let Natwest bank exchange it themselves when she pays by card? Is the bank's exchange rate likely to be better or worse than Wizz Air's?
Usually the bank exchange rates are better/lower, whoever I would recommend her to check the actual rates via internet banking.
The best and easier way to do that, would be simulating a transfer in another currency.