Take the Portsmouth -Caen ferry, lands at Oistreham, most easterly D - Day area. Then up river to Pegasus Bridge, first D Day organised action. Then west as you like. Do your WW2 homework first, you'll appreciate it much more. Don't even need the bike - the history is so thick on the ground. Aim to be at Arromanches at low tide to see the remains of the Mulberry harbours. Lots of good, cheap accommodation - I'd suggest chambres d'hote. And you can't really ignore the 900 year earlier bit of the reverse conquest.
More history per step than anywhere I can think of. Enjoy it.