Wow, if you close your eyes to a mostly visual medium, your perception changes significantly. That's some deep **** .
And yet you use it to justify your conviction that the skin colour is significant in whether someone can play a Scotsman in the 11th century. And not, for example, his accent, pronounciation, intonation, general posture, suitable costume or something else. That's some shallow ****.
Race is not the same as skin color.
What's in your opinion the difference when it's being used in Angloamerican countries nowadays? I'm genuinely interested if I'm missing something obvious that isn't racist.
It is acceptable for blackness to be seen as a defining trait and casting against is "whitewashing". But if a non-white actor is cast as a white character, then woah, woah, what do you mean race, we're all colorblind, what are you some sort of nazi?
Do you seriously don't see or just aren't willing to acknowledge the imbalance between more than a hundred years of whitewashing, which has repeately happened, the actual debate about the skin colour of casted actors and actresses, which is happening, and your hypothetical example?
But to pick up your Martin Freeman example: In a counterfactual world where the colour of the skin was never relevant for the cast of a role, there wouldn't be any outcry. In the actual world it is considered relevant therefore the (hypothetical) outcry would be justified and necessary in that case. When set in relation to the other case it can only be seen as either unproportional dishonest or (when combined with playing the Nazi card) plainly idiotic.
That whitewashing in movies isn't necessarily founded in anything else but financial considerations is clear. That a white cast did and may raise the revenue (and that artificially creating controversy by casting a black cast for presumed white roles may be based on the same motivation) because of the comparitevly increased popularity of many white actors and actresses likewise. That this can be and is considered to be an aspect of systemic racism in the movie industry too. Most or all of that has already be mentioned by Jacob (in far less words).
To reiterate my actual point, which didn't seem to come across clearly: to claim that a movie, be it a historical one or one based on a historical setting, is ahistorical (or "hilarious") because of the skin colour of the cast, while ignoring everything else that differentiates it from its presumed or actual historicity, is either an aspect of complete arbitrariness, ignorance or simple racism.
To resort to some identity-establishing aspects of communities is, in my opinion, a completely different discussion. Even if I acknowledged that the colour of the skin is one identity-establishing aspect of imagined communities (which I do because it is), I don't see any reason whatsoever to use such an aspect of societal reality to justify or condemn the cast choices in movies. Especially not when I'm convinced that the basis of such an identity-establishing aspect is not only arbitrary, but untenable, fundamentally false and damaging.