We'll be traveling from London to Brussells via Eurostar. When is the best time to change our English pounds into Euros? In London or after we get to Brussells? What currency does the Eurostar Train take on-board for drinks, snacks, etc.? Thanks to all for the previous Contact Lense info and advice.
You can simply wait and exchange your pounds in Brussels. The Eurostar food service takes both pounds and euros.
Plan to not have much left in the way of quid - spend it in the train station on the UK side for food or whatnot. And get Euro from an ATM at Brussels-Midi. That's my preference. ATM's are everywhere, they're always with an English option, work just like you're used to at home, and far more convenient (with better rates) than exchanges in train stations or airports.
We never change money in advance - use ATMs. Bring some pounds for an emergency (ATM eats your card, etc), but usually mine stays in my money belt and goes right back home with me. Only once in 7 years have we had a problem with the ATM system being down (or maybe it was our bank - Sunday night, here). We changed enough cash for that day and were good-to-go.