Of This and Other Worlds

That's an unusual take on it, to be sure! I can't recall anything in the book that would support the idea that the whole thing was in Aslan's head and that Narnia, the children, etc. were just imaginary, though. Perhaps it was the description of "layers like an onion" that made you think so, and made you feel that the heavenly Narnia and the heavenly England described at the end of TLB were less real than the "Shadowland" versions of them, rather than being more real, more glorious, bigger etc., as Lewis (or Aslan) described.