One of my students who spent some of his early years in England remembers
some localities which Twain might have had in mind.

South of London, in Surrey, is Horzel Common, where we sometimes went to
walk the dog.  One feature that I recall is a sandy pit that actually
came to mind when I read Connecticut Yankee.  Unfortunately there isn't a
cave nearby--not that I remember anyway.  This pit is also supposedly
where H.G. Wells, who lived in Guilford, I think, imagined the alien
space ships in War of the Worlds landing.  I hope someone can back me up
or straighten me out on any or all of this!---Kevin Hagan