for most merchants, if they charge in euros, your payment provider (either paypal or credit card firm or bank) will be charged in euros and your payment provider will convert it (at a poor exchange rate, ie not as good as the rate you’ve seen on xe.com) into pounds and add a small fee on for the conversion. You then pay the amount in pounds that the payment provider converted it to plus the conversion fee.
if you’re paying by bank transfer then you will most likely need to perform an international bank transfer, which probably carries a charge from your bank (£20 in the case of RBS), you tell the bank how many euros you want to transfer, they will work out how many pounds this is , and you pay this plus the transfer fee.
You can ask YT if they can charge in pounds and it will save you the crappy exchange rate that the bank use plus the conversion fee, but I doubt YT will offer this.