If you are successful in passing the initial sift you will get an email with a practice pack explaining what to expect and practice papers based on similar (but not exact) tests that will be carried out on the day. There are usually ten to twenty applicants in the tests and after each test those who fail are no longer required to sit the remainder tests.
Southern also use the computer based GB test which is available on the forum to download and practice and you will have to practice!! The key to passing the GB is pace, accuracy and consistency. After that, if you are successful there will be a driver/line manager competency based interview. Once again, if you pass this you then have the final competency based interview and finally a medical.
The forum has lots of useful info on what type of tests and possible interview questions you will be asked so have a good look around previous posts. Vicpaul is also a good source of info as he is a driver with Southern.