GCSEs taken in schools usually include a significant amount of graded ‘coursework’. This consists of projects, essays or other work done during the year which counts towards the final grade in the exam. This is ideal for students who find exams stressful. It can be encouraging to know that they have already achieved twenty to forty percent of the grade. However home educated students sometimes find it harder to manage coursework. This is because an independent person has to mark it. For this reason, some choose the IGCSE (International GCSE) exams which rely entirely on exams. This suits some students, but not others. You can read a lot more about IGCSEs, including information about several specific courses, at the Eddis Tutorial site .
Whether or not you apply for a review of marking or to be entered for retake should depend on a number of different factors.
Opting for a review is costly and may result in your mark going down as well as up. Also, the way reviews now operate means that marks will only be adjusted if there is a 'significant error' by the original marker.
Because of this, reviews are best used when you feel you did much better in the exam than your mark suggests. The best advice is to discuss the review option with your teacher before you commit to it.