Just out of curiosity, why do you want to do another as? I'd agree they'd prove useful for optometry but it could be stressful since you'd have 3 a2s to focus on too, so make sure you can cope
however if you do well enough, bio+maths can get you onto that degree, along with geography as a third.
As for being allowed, i'm not entirely sure (about the new spec) but if your school doesn't provide you with the opportunity, there's also distance learning courses you maybe can try. Maybe ask a teacher since they should know about spec changes.