Toptests is really good for free mock tests (I don't know the exact URL, just google them) and they have loads on there. I passed my theory just be doing them, reading my feedback on the areas I did poor on and re-reading those sections of the highway code and trying again. You shouldn't struggle with the theory test. They're a walk in the park compared to your A Levels and honestly, if you mess up your theory and fail it's okay. It's not like other exams where you get one shot at it. Failing isn't the end of the world.