The BBC don't give two hoots about canon so they've never decreed one.
Hardly any studio ever "decrees" a canon, because "canon" is a term coined by fans and critics to refer to the original core material as distinguished from its tie-ins, pastiches, and fan fiction. The creators of the original series therefore don't have to worry about canon and never give any thought to the word, because it only has meaning in relation to material outside the core work. The only time studios ever think about canon at all is when they do bother to issue statements about how the tie-in materials relate to the original work. For instance, Paramount/CBS's policy is that
Star Trek tie-ins don't count toward the canon, and Lucasfilm licensing's policy (but not George Lucas's own) used to be that the Expanded Universe was kinda sorta part of the canon up until the canon contradicted it, and so on. But usually canon is a concept that only fans think about. I guess the BBC is in that majority of studios that's never addressed the question one way or the other.