Dunno ~ It comes included in the fee for driving lessons ~ oh and the test costs £31 if I remember rightly.
PS A quick Google reveals loads of "Free Practice" sites but they all seem to wqant you to sign up/join up/commit to paying fopr lessons etc before they let you try the tests. I should add the test changes every time you do it. They don't ask the same questions or show the same things on the screen to each person so you have to know your theory and Hazard Perception stuff. They are changing the test in September. Unlike the discs from the Driving School the actual test is harder and involves gauging the level of hazard as you approach it ........which the discs don't. It is, she tell me, very different doing the test on touch screen to doing it with the mouse.