SOTO wrote: To briefly add to what's been said, the BBC cannot, legally, declare a canon, because they do not actually own all of it. Many prominent DWU elements are in fact owned by individual writers, or their estates. Even today, we can't use the Daleks without permission. That's why there are a large number of works covered by this wiki that are not licensed by the BBC, but which are licensed for their DWU elements.
Canon isn't a legal concept. The BBC can't decide that a canon exists because of Death of the Author. The intentions of the author/editor/copyright holder have no effect on the stories themselves after their publication.