The EuroStar is a train that goes to and from London, Paris, and Brussels. We took it from London to Paris.
It is much quicker and easier than waiting in line at an airport. You still have to go through customs and immigrations, but that is required anytime traveling in an out of a country and it was still much quicker with the EuroStar than it was at the airport. The EuroStar is also a lot cheaper than a plane ticket.
It goes really quick. To get to Paris from London it only took about 2 hours. You go under a tunnel when you cross the English Channel, which was really cool. I love tunnels! :D
I would definately recommend if you want to go to one of the three places from one of them (London, Paris, Belgium) that you take the EuroStar.