What exactly we take into account when we correctly predict the origin.
Don't count Poles in, then. Mongrel nation, and it shows. I am being told 'And I thought you were British', and my friend is considered either French or 'maybe Greek'. I do recognize Poles in London, by looks, but it's more because of clothes and behaviour, than physical features. Polish girls, famously beautiful, are just more classy, or preoccupied, than the English.
Common genes, I am not convinced. Slavs are considered a quite mysterious group as comes to origins. There might have been several waves of migration, from different areas, and so on. Nobody knows, actually.
We also usually die when we have above 44 degrees body temperature, British are more resilient and go over 100 and it is still considered OK
Now that is solid evidence, and experimentally testable :)
Hm, now we could heat people up, see who perishes, and then find their other common features, by way of strict mathematical analysis... brilliant, Nathan :)