Genetic studies have shown that they are, in fact, basically identical, and developed an idiosyncratic and quite homogeneous genetic identity in the Isles.

The largest genetic study so far makes me doubt that.

DNA studies do not indicate that separate classifiable subspecies (races) exist within modern humans. While different genes for physical traits such as skin and hair color can be identified between individuals, no consistent patterns of genes across the human genome exist to distinguish one race from another. There also is no genetic basis for divisions of human ethnicity. People who have lived in the same geographic region for many generations may have some alleles in common, but no allele will be found in all members of one population and in no members of any other.

They may have find some specific genetic markers in more frequency on 'the Isles.' Exclusive or as you put it 'idiosyncratic' appearance seems very unlikely.

Quite homegenous is a nice choice of words. We're all quite homogenous.