We base all coaches off who's sharing your room we do this to try to make sure that at least 1 person from your room is on the coach with you. However, it is not always possible if people have selected different pickup points and times. In some cases, coaches are organised by your committee.
Once coaches have been organised changes cannot be made due to the port of Dover requiring accurate coach lists so that they can check people through border control. Reps will make sure that you get on the other correct coach prior to your departure, if you do board a different coach to the one given to you, you may be stopped at Dover and not allowed to carry on the journey.