However, I'm sure you'd agree that a mixed race couple in Poland would experience difficulties because of Poland's social conservatism. btw my local city is Białystok, and if you keep up with the news, you'll know where I'm coming from.
Tolerance is very different from acceptance. You'll find that even in countries with the most heterogeneous populations there is always an underlying tension. You portray London as the paragon of integration. But this would be to ignore the reality of the situation of "sundown segregation" which you can read more about here:
Not only is "white flight" an ongoing problem in London and other areas of the UK but the British press is frequently forced to report (albeit very grudgingly) on Asian sex gangs and their specific targeting of white often underage girls around the United Kingdom.
As far as mixed couples go you can gauge the establishment's position in many different European countries which still have monarchies. Many who call themselves and are recognized as royalty may be willing to marry a commoner but none have yet crossed the Rubicon and married outside their own race. So what do you think this signals to those in the rest of society over whom they rule or will rule in the future?
I think you will find many communities and even entire countries around the world are socially conservative and not just Poland. Perhaps you are familiar with the Jewish term "marrying-out". But this isn't something limited to Jews. On several occasions now I've heard accounts given by non-whites of their conflicted feelings and even anger when their sons or daughters bring home a partner who is from another race or from a different religion.