I used a tour for the D-Day sites and would recommend them if your time is limited and not there for educational info from the guide, which was my case. But if you have the time, rent a car yourself so you can explore. I would have spent more time at some places but couldn't do to the itinerery.
On another note, I have a four day layover in London in a couple of weeks and wanted to head out to Bovington. So a question for the Bovington vets, what's the logistics from the Wool rail station to the museum? Bus, taxi, walk, hijacking, or what?