However... Imagine that you walk into a temp agency without an appointment. What could happen? You might be in luck, and the consultant might have a few minutes to spare for you. That means you can apply immediately. The question is then if you are ready for a job interview then and there. You should know that the consultant needs your CV in order to do the interview, and that you also need to have a basic idea of what kinds of jobs you wish to do.
If you don't have your CV with you, or you haven't figured out yet what job you'd like to do, then it's best not to walk in. It's better to prepare at home, or to make an appointment with the temp agency. That way you're sure the consultant is all yours during the interview.