I initially mentioned this text in a comment on the question. Although it seems this is not the specific text the OP was looking for, I'm also posting it as an answer since the text contains many of the features of that guide and others might therefore find it a useful resource.
Furthermore, as Pieter Geerkens has quite rightly observed in the comments above, comments are ephemeral!
While it was by no means an official guide for British diplomats, The Races, Religions and Institutions of Turkey and the Neighbouring Countries by the Scottish Liberal Party politician Sir George Campbell MP, published by the pressure-group The Eastern Question Association in the late nineteenth century, certainly seems to possess many of the characteristics of the document you describe.
That pamphlet does describe aspects of the stereotypical appearance of Balkan peoples, their political views, and makes some superficial comment on the relations to other peoples in the region, but lacks the table showing the different races, and their physical characteristics that you describe in the comments.