You can't have an official position that there is no canon, otherwise me saying that, I don't know, that The 7th Doctor was a serial killer in this sentence is technically as canon any episode of the show. The TV show is decently consistent and can hand wave away some of the more ridiculous things (like two Atlantis'). It sounds like people who just can't accept that Big finish is fanfiction like to pull some BS out of their rear. Its fine if you like Big finish, but it didn't happen. Things like the Doctor meeting Agatha Christie or having traveled with Sarah Jane Smith can and have be mentioned on the show years after the fact, but you're not going to get any episode mention the time The Doctor knew a sentient TARDIS in human form, or the time The Doctor fought werewolves with a companion who had already traveled with him in the future, or have an episode bring up that The Doctor used to travel with a talking Penguin, because as far as the show is concerned The Doctor never did any of that. There is canon because episodes follow after each other and reference each other. The show also specifically proves side materials as not canon by, for example, eliminating the looms (the Doctor was born like humans are, and had actual children not loom relatives), and even if that was incidental (and it probably was, being small plot points in different episodes), it still proves that some things didn't happen according to the show. Plus, the show will never purposefully go against itself. The Doctor will never claim that, for example, she/he's never been to Pompeii, or that she/he never had a granddaughter. So, there is canon, otherwise every episode can and probably would contradict every other episode.