It's unlikely that your credit card company will take you to the cleaners on this, but you need to know what its policy on foreign charges is, because you are likely to be using that card during the trip, right? Call the toll-free number on the back of the card and ask:
If I use the card in Europe, is there an extra fee charged on each trnsaction? If so, what is that fee?
Is there a %-based surchage on foreign charges? If so, what is that percentage?
What is the source of the exchange rate that will be used to convert foreign charges into US dollars? (You could try asking what the day's actual conversion rate is for GB pounds, but it's possible that the phone rep won't have access to that information.)
There are cards that assess no foreign-transaction fees and use conversion rates that are within 1% (or less) of the bank rate. Even if you aren't that lucky, you probably won't be paying much extra for your tour ticket. But if you don't like the answers you get, you might want to get a better card for use during your trip.
Similar questions should be asked of the financial institution that issued your ATM card, which you're also likely to be using in Europe. If you're going to be charged a fee every time you use an ATM, you might want to make fewer, but larger, withdrawals.