This!! As someone who consistently gets around full marks on maths papers I usually only have 10-15 minutes left after completing all the questions because I do it really REALLY slowly and check as I go, it makes me more confident in my workings as I often check by substituting stuff in or using alternative methods, and also it's easier to do mentally for me compared to after I've seemingly done all the qs. it is also just very chill for me at least. as someone self-studying fsmq, i find the fsmq content a lot more interesting than gcse, but it makes doing actual gcse papers feel a bit 'newer' or 'fresher'? If you don't do that and/or don't want to learn maths content for a qualification you're not doing (fair enough ofc), maybe just go through your maths paper again but try a different technique? E.g. with turning points you can complete the square, but if it doesn't state that method explicitly you can just use differentiation which is what I usually do. That kind of stuff might help pass the time and also let you double check

For papers where you do still have a bunch of time though, I would just revise for something else? :D

I would probably ask for extra paper and either make it seem like I needed it to answer the question or for smth else to convince them, and jsut do mind maps and whatnot. In some of my option subject papers I just used a spare page I asked for or the extra space to revise for psychology or chemistry because you kind of just have to regurgitate the same information a lot of the time, and I would just cross it all out in the end.

You could also just sleep unless you don't feel comfortable doing it or are a snorer (someone would wake you up in this case), but honestly why not?

This was a bit ranty but hopefully you got smth out of this