Not all universities will interview, and some will interview only for some courses. In general however, they want to know...
- If you are so disfunctional you'll be a pain to teach.
- Whether you have a genuine interest in the subject
- Whether you will contribute something to university life
- Whether you can think for yourself when not working to an A-level syllabus.
Pretty much anything else they want to know, they can get from the UCAS form.
Re: Salford - reasonable university, good aero side, run by some very capable and enthusiastic people. But, you can only apply to so many universities and personally, the idea of living in and around Salford wouldn't appeal greatly!
Re: Foundation year - if you can go straight into year 1 with the right A-levels, that must always be the best option.
Re: Maths, yes it'll be hard, but if you can get a B at A-level and don't mind hard work and (quite!) a few late nights of self-study, you'll cope.
G