Depends where you are going and what you want to see. In London there is the IWM, and HMS Belfast along with the Houses of Parliament, and Buckingham Palace.
Further afield, you have the RAF museums at Duxford and Corsford, both of which are worth visiting. Duxford, within easy reach of London, in particular for it has the museum dedicated to the USAAF. There is The Museum of Land Warfare there too.
I always get mixed up between Portsmouth and Plymouth, but do your research and if you can visit HMS Warrior and HMS Victory along with the Royal Navy Museum too. They are a bit further away, at least a long day out from London, as is Corsford.
: I get to spend 4 days in England and 5 days
: in Normandy next year, any suggestions on
: military sites that are "must
: see". Just looking for some help on
: highlights so I can put the daily itinerary
: together. Don't mind long days, it's a once
: in a lifetime trip (celebrating the big 50).
: Thank you in advance