Dear star123
I think your idea has some potential. There are a couple of business models you might look at Kip McGrath (I think that's how it's spelt) and Kumon spring to mind, you might even want to run a franchise from them.
I presume your tuition centre will run for a couple of hours after school and offer help and tuition to small groups in the subjects you mentioned. Individual tution can be expense for parents, so by offering a tuition center where help and tuition is done in small groups you, and parents, can benefit by economies of scale - e.g. 5 pupils, for £10 an hour is, from an educational point of view, viable, is more affordable than individual tuition and more lucrative than 1 to 1 tuition at £25 an hour, so go for it!
With the GCSE season nearly upon us, you may have missed the boat for this year, I'd focus on getting everything 'right' for a September launch.
Advertising - in and around schools is the obvious ones! In addition, talk to local schools themselves. If you have a good reputation, they may well be happy to recommend you - any extra tuition children have outside of school is of benefit to the school themselves as, if the child's results improve - even if its as a result of your tutoring! - it reflects positively on the school! (Who will no doubt claim the credit for the improvement!)
Jeff