In reply to Martin1978:
I've looked into this rail journey several times and come to the conclusion that its very expensive compared to flying by a budget airline or even a scheduled airline. It also takes a hell of a long time with no smooth connections. The cost of an overnight sleeper is even worse, and a lot more than breaking your journey by staying in n hotel.
Another alternative I've researched and actually done is ferry/tunnel and driving both via Calais and Sander or Bilbao. That again is two days and hellishly expensive in terms of fuel and tolls.
Air travel is so much cheaper (and quicker) that the alternatives are non starters unless you are wealthy.