You can also book over the phone on; 0300 200 1122
Or you can apply by post with the correct form (available from your instructor or the Driving test centres)
Please make sure that you are booking the correct test centre. There are 2 in Southampton:
Maybush - Southampton (Maybush) (must be in brackets)
Forest Hills - Southampton (Forest Hills) (must be in brackets)
When booking your test you will need:
your Driving Licence number
your date of birth
Your Theory test Certificate number (for practical test bookings only)
The date you passed your theory test
You will also need a valid credit or debit card to make the payment over the phone - The Driving Standards Agency's website gives information about how much the test will cost.