don't bother finishing your PPL if you're definitely going to Oxford. Double check with them but I doubt it matters.
this ^
Waste of money if you are doing whole integrated program and you may pick up some bad habits.
I think it has been said an employer would prefer to see one flight school all the way through your training.
If you want to fly on the weekend whilst at OAA then go for it..
and for your question on A level maths A* standard. It is not necessary, I don't have maths A level and my other A level grades are not that impressive but it's not stopping me.
If you want to do Aerospace Engineering then it is a prerequisite, but I don't think complex Logarithms and de Moivre's theorem will help you out at FL350