Just a couple of comments. My family took the Eurostar last year.

We booked out tickets March 29, 2013 for a July 5 crossing and paid $68 for the adults and $65 for the teen children. This ticket was quoted in dollars. So buying in advance really does save money.

It is sometimes possible to book Eurostar tickets through the German bahn.com site if you are going to or from Western/Southwestern Germany. Just plug in cities. My daughter and I returned to the Continent on July 10, London to Koblenz. This was on the Eurostar, with connections in Brussels and Cologne. The cost for two first-class tickets was 204 Euros, which included reservations. I made this reservation on April 15, so almost three months ahead. I suspect that in some cases it might be possible to do throwaway ticketing.

I think I may have had some extra savings on the trip because German Rail does not charge for one 14-year-old or younger traveling with a parent, but I did not get the full discount that I usually get. So my daughter did have to pay something. Usually the child travels for free with an adult on any German ticket, including international trips. But anyway, the point is that if you are going in and out of Germany, see if you can book the whole trip on bahn.com instead of piecing together the Eurostar with another ticket. You might save some money.

Finally, when planning itineraries, be aware that there is an overnight train-ferry-train between Amsterdam and London that operates at somewhat reasonable prices. You can book a cabin, sleep overnight, and kind of enjoy what amounts to a crappy cruise ship. I haven't been on this, but would like to.

I realize none of this really applies to the OP's trip, just throwing it out there for anyone planning a trip in the future.