How many know that British territory was indeed occupied by the Nazis in WW2?

Occupation of the British Channel Islands of Jersey and Guernsey

German underground hospital built with imported slave labor

. . . "Repressions and deportations:  The Germans showed in the Channel Islands what they would have done to British Jews had they occupied Britain. There were only a small number of foreign and British Jews in the Channel Islands. Most of the Channel Island's Jews evacuated in June 1940, but officials did not permit foreign Jews to leave for Britain. Three Jewish women of German and Polish nationality, Therese Steiner, Auguste Spitz, and Marianne Grünfeld, had fled Central Europe to Guernsey in the 1930s but had been unable to leave Guernsey as part of the evacuation in 1940 as they were excluded by UK law. They were deported to France in April 1942 to be later shipped to Auschwitz where they died.^[6] There were 17 Jews in the Islands when the Germans
arrived. Soon after the German occupation, officials issued the first anti-Jewish Order (October 1940) which instructed the police to identify Jews as part of the registration process. Island authorities complied, and their registration cards were marked with red “J”s; additionally, a list was compiled of Jewish property which were turned over to German authorities.[Fraser] Placards were placed on Jewish shops in German and English –’Jewish Undertaking’, and in turn most of the local Jewish populace had to sell their businesses. The process developed differently in the three islands. Jersey Jews and 22 Jersey islanders died in concentration camps. However, local officials made some effort to mitigate anti-Semitic measures by the Nazi occupying force, and as such refused to require Jews to wear identifying yellow stars and had most former Jewish business returned after the war. Nevertheless, police officials on both Jersey and Guernsey complied with the requirements of civil registration, including Jewish identity, imposed by the Germans (although officials in the registration department also procured false documents for some of those who fell within categories suspected by the Germans).^[6] Extra curfews were imposed on the Jewish population of the islands, and most were forced to stay inside for the large part of the day. The Duquemin family was deported as well, but survived the war.[1]

"However, the anti-Jewish repressions were not carried out systematically. Jews of British citizenship were less likely to be persecuted than foreigners. A number of well-known Jews lived through the Occupation in comparative openness, including Marianne Blampied, the wife of artist Edmund Blampied.^{[6] " . . .}  Full article here. Photos added by TD

Who expected to see sights like this?

The Guernsey Press: The German Occupation of the Channel Islands from the summer of 1940 to 9 May 1945 now forms an important part of the history of the Second World War.

. . . "These islands were the only British territory to be occupied by the Third Reich and for islanders it was a traumatic experience, and today a well documented one.
"In Guernsey and Jersey part of the population evacuated to the United Kingdom. In Alderney all but a few islanders left their homes for the mainland, and in Sark the population remained intact.

"All the islands suffered an influx of German troops, and the main ones were heavily fortified with reinforced bunkers and light and heavy guns. Hitler considered this as part of his Atlantic Wall, which was built to withstand any invasion by Allied Forces. As Hitler's war machine swept through France, fears were growing in the Channel Islands regarding a possible Nazi invasion.British troops were pulled out of the islands, which were given an open town status." . . .

German occupation stamps. The occupation forces added this Swastika image to the regular stamps.