Although there are differences, people across northern Europe are not all that different. Fst across the region is surprisingly low. This extends to appearance: although you can notice that Danes don’t look just like Scots, there is overlap, people from group A that could superficially pass for members of group B. Superficially: looking at appearances, not at autosomal DNA. All because the whole area is ( to a large extent) the product of Indo-European invasions, and before that an Anatolian agricultural expansion. The recipe is not all that different over that region.
I think that it would have been harder to infiltrate someone from China into MI6 without anyone noticing.
On an unrelated note, occasionally we have had people suggest or wonder about whether northern Africa, north of the Sahara, had much admixture from sub-Saharan blacks back in Classical times or earlier. We now know, from ancient DNA, that moderately ancient Egypt had significantly less SSA admixture than Egyptians today, and it is reasonable to think that this was probably true of North Africa as a whole. I was already sure of this – because although nearly all contemporary North African populations have a fair amount of SSA admixture ( ~20%), a few have almost none ( < 1%). Key fact: there are several ways to mix two populations, but (without using advanced genetic technology or waiting several tens of thousands of years, in the case of mildly incompatible subspecies like AMH and Neanderthals) there is no way to unmix two populations.