Below is a link to the Driving Standards Agency website, these are the people who conduct the theory and driving practical tests.
When you feel you are doing well with your revision it would be a good idea to practice the two theory tests that are available as this will give you a good idea if you have prepared fully to sit the theory test.
Please remember that the theory test consists of two elements the theory part and the hazard perception, both parts must be passed to recieve an overall pass.
PASS MARK FOR THE THEORY TEST IS 43 OUT OF 50
The link above can also be used to book your theory test online, this link takes you to the Driving Standards Agency website, this is the cheapest and best way to book your theory as there are no extra booking fees.
You will need your valid provisional licence and a credit or debit card for payment, the cost of the theory test is currently £31.00
To book click on the link above then on the new screen scroll down to more useful links and click on book your theory test online.