Most people I know currently applying for ChemEng/studying ChemEng took the same combination of subjects: Maths, Further Maths, Chemistry, Physics and/or Biology.
I would say that a Physics AS would be enough. In your case, I would take Biology instead of French as an extra science subject will put you in a higher position to be considered. However, French is regarded as a preferred/traditional subject by the RG unis. As long as you have Maths, Chemistry and Physics, that's pretty strong combination already.