Deutsche Bahn have some regular cheap train ticket offers that they keep well hidden. They're intended for people in Germany, but can be booked in English in the UK and there's a UK callcentre.
London Spezial: London to anywhere in Germany from EUR49 one-way.
Take the Eurostar from London to Brussels, change to a German Inter City Express train to Cologne, then German trains after that. This includes German border stations like Salzburg and Basel Bad. First class from EUR99 each way.
Europa Spezial: Germany to Belgium/Netherlands/France/Austria/Poland/Switzerland/Luxembourg/Italy/Denmark/Croatia/Czech/Hungary/Slovenia/Slovakia from EUR29 one-way, or EUR49 in first class.
You can even put in two stopovers of up to 48 hours each for no extra cost! And they don't have to be on a direct route.
Prebooked trains only, book up to 90 days in advance. DB's site doesn't have a 'shopping basket' so they charge a EUR3.50 postage fee for each return journey (returns can start and finish in different places, for any number of people). So to book more tickets, either book on the phone (0871 8p/min) or pay postage for the first ticket into Germany then book the rest to collect from a German machine for free. Seat reservations on ICE trains cost EUR2 each, but aren't compulsory.
EDIT: For an English booking site, just click on 'Go to deal' to the right of this text.