H. G. Wells discusses the former unity of Britain with the rest of Europe in his "History of the World". If memory serves. :')
... for then the land was higher and stretched out farther than ever, and it is said the Thames ran into the Rhine, "Ireland, England, and the Continent being united, the Thames was a tributary of the Rhine," page 274.
http://www.stempublishing.com/authors/darby/NOTESCOM/44014E.html