Computer Science with Mathematics - advice needed

In applying for a joint honours, both schools must approve the application. For the school of mathematics, do they take into account the fact that the applicant is doing joint honours? I was checking this link:

http://www.thestudentroom.co.uk/connect/university/university_of_st_andrews

From this it seems the standard of successful applicants doing single honours mathematics is really high, added with the fact that I didn't do further mathematics, have not done STEP and my personal statement is stronger on comp sci than maths, I'm worried that my application will get rejected as result. St Andrews is literally the only university I want to get into.

Grades: GCE: A* Maths A* Chemistry A Physics GCSE: 4A* 4A 2B (A* Maths)

Firstly, If my application is approved by the school of comp sci but not the school of mathematics, would I be completely rejected or would I be able to get a place for just comp sci? Secondly, if not, then would I be able to apply for both 'Computer Science' and 'Computer Science with Mathematics' at St Andrews now to avoid this? Or is that not allowed or would actually hinder my chances?

I would appreciate any help, thanks.

In your first two years you normally do modules in your chosen subject plus modules in two other subjects. If you apply for Comp Sci perhaps you could pick up maths on entry as one of your other modules. http://www.cs.st-andrews.ac.uk/prospective-ug/cs-programme-structure
I would imagine - as I know happens in other subjects - you could switch to joint honours later, provided you have taken all the compulsory modules for both. It's a very flexible system. But you might want to check that with the admissions office.
My son applied to two different courses at Durham and got offers for both. I have a feeling from past years that some people who applied for two related courses at St Andrews were told they would only be considered for one and the university decided which. But again, that's something the admissions office can advise on.