And then what are you going to do?
One of the things to remember about casting people is this: Much of the time, there is a point at which your casting choices become somewhat arbitrary. I've cast or helped cast theatrical productions, and often you hit a point where neither Actor A nor Actor B are truly "better" than one-another. You have to make a choice, so you go with your instinct, and it's probably a bit arbitrary.
If the Doctor Who
team were to make an effort to find the best actors out there who are not white Englishmen, obviously they would have to pick one person, and at some point their choice may well be somewhat arbitrary. That's okay. The goals would be to find a great actor, and to break out of the mindset that one should see "white English male" as the "default" setting.
So if they then choose the white Welshman over the black Englishman this is ok because at least they've broken out of the default setting of a white Englishman? I mean seriously is a white Welsh or Scotsman really that different from a white Englishman?
I'd love to see the casting call as well, and I'd love for a white English actor to then take, perfectly justifiable, umbridge.
No, it needs to be the best person for the job or the person cast risks having no credibility, and I say that as someone who can probably think of more non white actors who'd be the best man for the job than white ones!