As for a German retailer I am quite fond of mindfactory.de/compuland.de (basically the same guys). Good prices and a great customer service. Never bought a prefab comp from them, though they do offer them. You can also select your hardware and let them build your PC and test it - might be an alternative when you put the shipping costs from the UK to Germany into calculation. Both shops often have various special offers, like free shipping and other stuff.
A good price comparision for Germany is
http://www.heise.de/preisvergleich/ or
http://geizhals.de/ (same stuff, but the first link is from Germany's best IT site, heise.de) You can also search the site for prica comparisions in Europe, but the amount of retailers in the EU-wide search is pretty limited.
Hope the guards in Belsen will let your parcel through!
