According to this Wikipedia article,

""" In his 2004 chapter "A Mythology for Anglo-Saxon England", Michael Drout states that Tolkien never used the actual phrase, though commentators have found it appropriate as a description of much of his approach in creating Middle-earth. """

So this is critical interpretation and not something he literally said he intended. His quotes (also in the article) suggest he was drawing from English and Norse mythology, of course, but not that the cosmology of Middle Earth is solely English.