I think that at this stage, practice exams are still too early. However, exposing yourself to questions/practice exam questions relating to a particular topic you study will really help you in your expression, and practice makes perfect. I'll be doing Unit 3 Practice Exams for Bio at around mid-year, and I'll be then doing unit 3+4 exams at the end of the year. Further Maths is a bit different because conveniently, its divided into each module, so as I finish the module, I do about 2 exam 1s and 2 exam 2s, and so I've gotten a little hint of the structure/types of questions asked.
But I mean you can always spend time (now) really consolidating that information and ensuring that you don't depend on just types of questions in the exam. Finish off each topic knowing you have completely consolidated it to the best of your ability, before you start doing practice exam questions/diagnostic tests.