Even if you get As and Bs in your mocks, you still have a very good chance to be awarded A*s in the majority, or all of your subjects, as there were people who did so in my year. But the most important thing is that if you can get the grades you want when doing past papers, and if you're preparing so much that you've done work in Year 10 summer, then there's not really much reason you can't get those A*s overall!
If you have an A* in all your English coursework, you probably won't necessarily need an A* in the exam, as I think the only A* I got in an English exam was AQA English Literature Unit 2 (Poetry), but I somehow managed with A*s overall for both Language and Literature!
Just remember, that unless your specification has completely changed, then it's very much the case that the exam you take on the day is almost exactly like every other past paper you've done, but with it just having the examination's date on the front as an exception. You'll be more prepared than you think you would be on the day, and I find that I tend to go into super-adrenaline mode whenever I took any of the exams - mainly English and History.
You can't really choose what you want to do at A-Level until you've attended a few sixth form open evenings/days to find out what they're about! But you'd have to decide probably around about after your mocks, so that you can put them down on an application form, before the deadline to apply to colleges. Even then, you can change your mind when you have your interview(s) with your applied colleges at around February/March, and you're allowed to change your mind again before you enrol!! Of course, after you enrol, you'll be given a limited amount of time to change courses if you're really not liking a particular A-Level, but at that point, there's not really any guarantee that there'll be any spare places available. But overall, you have plenty of time, and all you should really have at this point is a vague idea of what you'd maybe like to do.
You're very welcome for the advice, and I hope your GCSEs go well too!