I read the first page of the thread.
First you have to identuf wht you arent turning up, but if you decide not to, then it is your choice. If you have good reason you might notify the school that you wont be turning up and theres nothing they can do about it. they might dislike it and they may make things more difficult for you in terms of resits etc. its best not to inconvenience them, so it would be better to confirm it with the exam officer. I really dont think it would throw the school into panic.
In terms of impact on you, then you just need Maths and English plus three others.
If you want to go to Uni then you might wnat to get more at good grades, but its really your A levels that count. In which case if you intend to sit A levels, then make sure you have the relevant GCSE.
Persnally I dont see any point in taking an exam you genuinely know you will fail. I would rather resit.