I was doing research on capital ships of the Royal Navy and I was shocked to see that HMS Dreadnought (Or really any dreadnought class battleships) wasn't around as a museum ship. I also discovered that it was the case for most eras of their capital ships.
Granted, I'm an American and we have tons of ships from the all of our eras in ports all over the country but I thought there would be more capital ships floating around as museums in the UK due to the Royal Navy's history.
I have answered this before here, with particular reference to the post-WW2 era. The same arguments generally hold true for the post-WW1 period in which Dreadnought was scrapped, though here we must also acknowledge the existence of the naval treaties of the 1920s and 1930s. These held the Royal Navy to a strict total tonnage limit in capital ships; anything over this limit had to be scrapped or completely demilitarised. Such ships could not be preserved - any preserved ship might be kept in a condition where it could be reactivated, and hence serve as a way to avoid the treaty. The Washington Naval Treaty led to the scrapping of most of the RN's remaining fleet of dreadnoughts (only the super-dreadnoughts, armed with 15in guns, were kept), while the 1930 London Treaty did for the remainder.