LONDON — When Britain made its formal exit from the European Union in 2021, the move attracted some unlikely fans — among them, South African citrus growers. With the U.K. out of the bloc, it was free to implement its own rules on imported food, including relaxing stringent EU controls on citrus fruit. Today, thanks to Brexit, citrus fruits such as lemons and oranges can be imported to the U.K. without a plant health certificate and without the need for inspection, making the country an