Former London Times journalist Guy Walters' thriller "The Occupation" (2004) describes the development by the Nazis of missile launching batteries on the island of Alderney, which was just a few miles off the coast of NW France. According to the plot, when fully ready the site would be capable of saturation-bombing London at the rate of 300 rockets an hour.
The Daily Mail published an article on the Alderney fortress last year - "Hitler's British Death Island", 5 May 2017 - reporting that at least 40,000 imported slaves were killed in the process of constructing the site, which according to former military officers Richard Kemp and John Weigold was to target the southwestern coast of Britain from Weymouth to Plymouth with sarin-filled warheads.
However the newspaper's claims were swiftly contradicted by the Aldernese historian Trevor Davenport. Even the Mail's story admits that such a project would have been against the Germans' policy regarding V1 emplacements, as being more vulnerable to naval gunfire and commando raids than mainland installations.
The real underground tunnels can be seen here: