[A_Brandybuck]

I do not think, that Tolkien really wanted to attach his work as the 'mythology' of England, that he wanted to write the past of 'real' England.
I rather think, that he wanted to write a mythology, how a mythology 'could' have been in the past. A mythology, which fits in the whole context of Germanic mythologies.
The main reason, which let me think, that he didn't want to attach the mythology to 'our' world, is his Christian belief.
Although he excluded the religious aspect in his works, there is a God in it, Eru, and it is not the Christian God. Attaching this mythology would mean, that he agrees in the existance of another God in the world. And this would mean 'Blasphemy'. We all know, that Tolkien was very religious, and my personally meaning (or rather interpretation) is, that he would not dare to it.

Tolkien was sad, that there aren't a mythology for England, because he loves the other Germanic mythologies. But he knew, that these mythologies were grown over ages and include the culture of a people. I would say, that he would never arrogate, that he is the one, who gave England a mythology, because of his knowledge, how the other mythologies come into existance.

Consequently he tried to write a mythology for England, how this could have been, but not is.