Cycle trip from South of England to Germany

Hi!
I was wondering whether there are any experienced cyclists around... I am planning a Spring trip with a friend of mine and we want to go from Portsmouth (UK) to Paris, then Brussels, Amsterdam and then Frankfurt. Has anyone ever done a trip like that? We want to cycle offroad and camp on the way.
I am writing this thread to get any type of feedback and suggestions, so any comments are welcome.
Thanks!
Jen.