I know there were a handful of Wendy's in London in the 1990s and,
according to this blog post at Imgur, there were a few Wendy's franchises in mainland Europe as far back as 1979. In 1988, a man named Raymond Warrens opened an unrelated fish-and-chips shop called "Wendy's", named after his daughter, in Goes, Netherlands. The blog entry doesn't say when Wendy's, the American fast food chain, completely closed shop in mainland Europe, but notes that the brand was not in use in the Benelux countries (Belgium, Netherlans, and Luxembourg) by 1995, when Raymond Warrens snatched the "Wendy's" trademark for Benelux, which, apparently, also makes it very difficult for Wendy's U.S. to operate under that name anywhere in the European Union, which the Benelux countries are also member states of.
The last British Wendy's closed down in 2000 but Wendy's has shown interest in trying again,
operating a pop-up restaurant in a bar in London on a single day in July 2017.