The Map Room at Southwick House, Hampshire, still houses the original map used to plan the assault on D-Day (otherwise known as Operation Neptune).
Explore the map at https://dday.nmrn.org.uk/ (UPDATE: This seems to be having problems, sorry) and other resources of the National Royal Navy Museum at https://collections.nmrn.org.uk/search/simple
I couldn’t get the map to load past the home page. I whitelisted the site in my ad blocker and tried two different browsers, Chrome and Edge.