The Channel Islands
Collective name for a group of islands off the French coast, the largest and most significant of which are Jersey, Guernsey and Alderney. They are British "Crown Dependencies" (i.e. not part of the "UK") but since they are territorially separated from the British mainland they are not covered under the heading Great Britain here. They are actually closer to and thus more easily reached from France (esp. from St Malo). But as they are so English in character, it wouldn't have made sense to cover them under the heading of France either (which would have been politically incorrect anyway, in the literal sense). So they earn themselves a separate chapter. A short one, for the time being. I have only ever been there once, many years ago for a brief day trip from St Malo, but I realize that at some point I will have to go back for a more comprehensive exploration.
The main reason for dark tourists to head for these islands is the fact that there are numerous sites related to the period when during WWII the islands were the only British territory ever occupied by Nazi Germany. Those few years have left a significant mark on the islands, also in psychological terms (having "overcome" the Nazi period is a major component of the islanders' collective "identity"). There are physical remnants too, and some are quite heavily promoted locally in terms of tourism.
Remains of fortifications are numerous and visible almost everywhere, there are also underground tunnel systems dug during that period, and several war/occupation-related museums. If you wanted to pick out one major site it would probably have to be the Jersey War Tunnels – German Underground Hospital Museum, which has become one of the premier tourist attractions on the island (open daily March to November 10 a.m. to 6 p.m., last entry 4:30 p.m., admission 11.20 GBP, seniors 10.20, students 8.20).
The Channel Islands' war-related sites are pretty harshly criticized in Lennon and Foley's book "Dark Tourism", esp. for the commodification/commemoration of the war period being heavily tilted. While resistance against and liberation from the German occupiers are celebrated at the various memorial sites, darker aspects such as collaboration and the deportation of the islands' Jews are apparently barely touched upon. I have also heard lectures by other academics (including one actually from the Channel Islands!) making similar points. I will report back here from first-hand experience once I've been back and seen those sites for myself.
Location: on average some 20 miles (30 km) west of Normandy, France, and between 40 and 75 miles (70-120 km) north of the port of St Malo; and around 90 miles (140 km) south of the Channel coast of England, Great Britain.
Google maps locator:[49.2102,-2.1542]