I don't understand why you are applying for Business Finance if you want to do law, particularly if you are willing to take a gap year in the event of no offers. Maybe look at AAB unis for law instead? It could be difficult to write a PS that was convincing for both, and this could weaken your chances accordingly.
For your PS, maybe use the reviewing service on here?
Going to a state school isn't an advantage per se but Oxford will contextualise your GCSE results so they will see if you did better than your school average.