I'm a bit puzzled why a generalisation about the Britiish having a tendency, rooted in a national culture, to be racist, is particularly different from a suggested generalisation about Pakistan's culture playing a part in certain offending patterns among a tiny minority.

Neither might in fact be valid, but even so that would not mean that they were in themselves to be seen as an all compassing attack on the communities involved. The same would apply if either generalisation were valid.