I'm sorry that you're disappointed with your result, but unfortunately unofficial mark schemes really aren't a reliable indicator of how you've done in an exam, particularly where there's some subjectivity (for example, the awarding of marks for showing your method), and can cause more harm than good in the long run, as you've discovered. You can always request a remark, but bear in mind that you'll probably need to pay for this, and your grade can either go up or down as a result (though they most commonly stay the same).
The good news is that unless you're dead set on taking A level maths or studying medicine at Oxford, all you really need to do in GCSE maths is pass, which you've done, so there's no need for you to resit it.