It is not quite as trivial as the publication date of Selling England seems. Reason enough for Alexander Sturm and Peter Schütz to do some more detailed research. You can find the result here.
Mark Bell reconstructs the genesis, content and afterlife of the album "The Lamb Lies Down On Broadway" in his new book. Martin Klinkhardt evaluated the book.
The album was released in 1971 at a time when Genesis were still far from being a big name in the scene. Accordingly, it is difficult to prove the specific release date.