-- the main protagonist in the book was a male domestic servant for much of the story. And English, at that. Doubly unlikely.
Peter's enslavement was a part of the punishment for his "crimes". The English are Aryans, albeit non-Germanic Aryans (they are "mixed-race"), but Peter is reclassified as an Untermensch. As Dr. Lederman explains "Your actions have shown your true blood", ". . . you must have some Jewish or other foul blood in you, as do so many of you English."
He was employed initially as a shop-worker as part of an experiment to see whether criminals could be successfully reformed by methods that in OTL would be called behaviourism. In order to see whether or not their methods had worked, Peter had to be employed in a position of responsibility. Karl's wife wanted a domestic servant, but he couldn't afford one, so he pulled rank in order to take Peter from his previous employer at a low price.
So this Englishman ending up as a domestic servant was the result of a series of fluke events, not something common in TTL.
Nazi ideology was mad, but they took it seriously, and they always regarded the English as they did the Dutch or Scandinavians; ie., essentially racially identical to Germans.
(For once, they actually had something; genetically speaking the English _are_ Germans/Netherlanders/Frisians. Ghu alone knows what the "Race and Resettlement" bureaucrats would have made of the Welsh.)
Actually, the genetic contribution of the Anglo-Saxons to the modern English race is negligible. Both the English and Celtic peoples of the British Isles are genetically the descendents of the Pre-Celtic ancient Britons.
"Nordic" occupied countries were treated differently from, say, Slavic ones and the difference would almost certainly have become more pronounced if they'd won. The Nazi higher echelons regarded them as potentially valuable sources of human breeding-stock to augment German numbers, which was a eugenic obsession of theirs.
The highest probability is that they (and their populations, minus the Jews, of course) would eventually have been incorporated in Greater Germany and treated as "honorary Germans", with intense attempts to Germanize them linguistically and culturally and to recruit them as settlers in the East. A start was made on that during the war, in fact. It would probably have worked, too, if they'd won.
IIRC, Nazi policy in TTL in the years after the war *was* to Germanise the English - Peter's parents and brother are "Germanised" Englishmen - but the attempt failed due to German favoritism towards other Germans and due to the dislike of the English population at large to having a foreign culture imposed upon them. At some point, Nazi ideologues decided that the persistant troublesomeness of the English must be due to their being of impure blood.
The sending of young British males to Germany to work as labourers - seen at the beginning of the book - was something the Germans had planned to implement after the conquest of Britain. It was concieved as an attempt to Germanise the British but in the book it has ended up being more about supplying Germany with cheap labour.
According to the map at the end of the book, Britain *is* part of Germany, classed alongside northern France, Poland and presumably Denmark and Norway as a "New Reich" area. Ireland is an "Autonomous Region" and Sweden is a "Protectorate".
I may be imagining this, but I think it says somewhere in the book that the Germans take very little interest in Wales.