http://www.raileurope.co.uk/
Rail Europe specialises in selling tickets and passes for travel throughout Europe by train. Available to buy online or via their call centre.
http://www.seat61.com
This website will tell you the best train times and approximate fares from London to major destinations all over Europe.  It will tell you what the trains are like, and the best way to buy your ticket.
http://www.europeanrail.com/
As independent specialists for continental rail travel, European rail will guide you through the labyrinth of fares, routes and timetables allowing you to select the itinerary right for you.