Someone posted on the crackpot theory that its Lexa Doig on the anon meme.

I am still holding out hope for Paterson Joseph as he's awesome....but whomever it is, they will be awesome.

ETA: Supposedly The Guardian is saying its between Joseph, the other guys who's name I can't pronounce let alone spell, and Ben Wishaw (who I just went and looked up and he looks WAY too much like Tennant for it to be serious).