In England and Wales, you can ask anyone to prepare planning drawings, even an Interior Designer! In theory, your plumber can do it. Even you can do it! The same applies to submitting the planning application. You don’t have to be an architect or have any qualifications to perform these tasks.
However, it is recommended that you ask someone with the necessary experience to prepare the drawings and guide you through the planning rules, as they are complex and sometimes open to interpretation. In this regard, an experienced architectural designer is probably the best person to appoint.