As far as I'm aware Exeter, Imperial and Newcastle don't require a grade 6 at GCSE maths/don't look at gcse's for pre-interview selection. Others are unlikely to be lenient with the grade unfortunately due to how their systems are set up. Most have 6 as a minimum entry requirement for maths and will automatically reject those with anything below it and won't consider how close you were to the grade. You may want to look into resitting maths as would give you a lot more options to apply to. Hopefully this helps.