It all depends on whether the 7 and 8´s were grades achieved when coasting or the result of consistent hard work. If you were honestly giving your best, then imo the A* will be harder for you than for someone getting 9’s at GCSE. But I am generalising. Some people discover themselves and their academic talents later on in their studies. The fact that you have narrowed down the number of subjects and can concentrate all of your study time on subjects that interest you may make a big difference. As long as you are sure that the foundations of your chosen subjects are really acquired and If you are motivated, organised and conscientious in your approach to « A » level study anything is possible although probably not easy.