What would be the best option for exchanging pounds for Euros? Where? When? I'll be traveling by Eurostar from London-Paris.
How many are you expecting to have?
If it was a small amount, I would hold on to them and get more cash out with my debit card from an ATM when I got to Paris. If I had a large amount I would probably exchange them for Euros at a bank...
Just remember, the more exchanges you do, the more money you loose.
As I recall there are two ATMs inside the waiting room at the London Eurostar. Kinda over to the left from the security line entrance (near I think it was a coffee bar). One is for Euros and one is for Pounds. Maybe you could put in pounds and get out Euro's from the appropriate machine. Never tried it myself as I just bring home any extra money for the next trip. If you're not returning to London just spend down until you've got a very small amount and donate it to the "Children's Fund" or whatever on the flight back to the U.S.
How many pounds will you have? If just a few buy snacks at the station or on the train. There is a bureau de change at the station too.Last time I took the Eurostar, I saw a Euro ATM. Gare du Nord will have the same options too.