While the FSMQ does have benefits for college, I remember that I was put onto the course at my secondary school and taken out of some GCSE maths lessons for it. I found it very challenging, and I was definitely not good at it. A few people got low marks in the mock, but I definitely got the lowest - it was either an E or a U. I spoke with my teacher about it and we decided that the best thing to do was to keep my GCSE grades high. At the end of the day, no matter how useful the FSMQ is in terms of making you look good, your GCSE is what matters.
Unless your parents are pressuring you in any way to keep up with it, then I would just explain your decision to them and why you feel it would be better to focus on your GCSE instead. They may not be pleased about it initially, but over time they should come to see your point of view. Eventually they should realise that getting a high GCSE should be a priority over the extra qualification you could gain here.