Hi - A family member teaches at Chester Uni but on their Musical Theatre course and I have a friend whose son is studying on their Popular Music Performance course. I'm currently helping my son to research music tech courses and I have a daughter who is training to be a dancer/musical theatre performer.
For entry you need to pass an audition (they are looking for around Grade 8 standard on an instrument or voice but you don;t need the formal qualification) or have 112 UCAS points. To be honest most applicants will have either have been studying a Btec in Performing Arts/Music or Musical Theatre or have A levels and performance experience. The prospectus says the ability to read music is desirable but not essential.
Very few make it as performers, most musicians have what is known as a portfolio career and many university courses are designed to prepare you for that. So you might combine gigging with teaching with working on an arts project/admin side of things or composing/arranging for others or some aspect of music tech. For this I would say you need a knowledge of music theory, and have good muscianship * analytical skills. I'm not sure what you mean by attending castings? I wasn;t aware that there were really music castings as such. Theatre castings yes, for which you generally need to have attended an accredited college with an agent showcase and be submitted by your agent. There are open calls but its trained performers that producers are looking at. Most musicans I know tend to start out small, putting on gigs, advertising wedding bands, maybe getting pit work (for which you need to be able ot read music) etc etc
What is is that draws you to Chester particularly?