Regardless of whether you can or can not find a course/school that you can enroll in without a C in Maths, I would strongly recommend going and getting a C or better before you start commercial flight training. Not only will it open more options for you school wise, it will also help you through the course (how much depends on who you ask probably) and will (arguably most importantly) open a few more doors whenever you are looking for a job afterwards.
Why someone with the licence and ratings required needs grade C at GCSE to work for some operators I don't know. It may just be a way of ruling out some more applications, but you may as well go and get the grade now and not leave yourself short of options.
I imagine you could do a Maths GCSE fairly quick if it was all that you were focusing on?