I have family in Normandy so go there a lot. I'd recommend the ferry. Brittany Ferries sail from Portsmouth and Poole to either Cherbourg or Caen. Caen is nearer the British Beaches and there is a huge British war cemetary there as well.
If you travel up a bit towards Cherbourg there are loads of interesting places - Ste. Mare Eglise (one of the main drop off points for the American paratroopers) is a nice little town. If you're interested in the German side of it too I can recommend a visit to a German War Cememtary in a place called Orglandes. Its off the beaten track a bit in amongst some lovely little hamlets near the town of Valognes. It is signposted from the N19 motorway. Its actually very moving when you look at the graves and realise that almost all were killed in the space of about 5 days and they were almost without exception the very young (some only 16 and one I saw who was only 14). Puts a human face on the enemy and well worth a visit in my opinion.
Have a good trip.