damn thats tough.
firstly, the council or government should be able to help you get a school.
secondly, i assume this must be under some sort of special circumstances that they should consider. if you briefly mention what happened in your reference/personal statement and the grades you were predicted, or email the admissions offices, i'm sure you will be ok.
also, make sure you want to replace chem with history as you're clearly strong in chem. have you considered doing 4 a levels? its not necessary but if you think you can manage, as you've already done half of 3 alevels, it will defo help when applying to oxbridge but not other unis so much.
all the best