Tanya,
The best time to take your driving test is whenever you feel ready to sit the exam to be honest. However when you have done a bit of practical driving, then the Highway Codes that you've learnt make more sense and helps you to better understand the signs and rules about driving.
From what you said, I think you're ready for the
driving theory test and you should do it now rather than later. Good luck to you! And don't forget to let me know what it went :)