I do not think the A levels will be of any assistance as the maths and physics needed is very basic (from what I have seen so far at stage 1 and what I have heard of stage 2)
Someone previously mentioned they try to get 3500 - 7000 to sit stage 1. I do not know if an "applicant" counts as people who sit stage 1 or everyone that applies including those who get knocked out at the on-line stage.