Talk to computer science. Guage your chances and then reapply next year. They may have said no because they are full or because you didnt meet the requirements.
I exceed their requirements (their reqs are A*AA, I have A*A*A*)
Not exactly expecting too much, my friends at Bristol have successfully done subject swaps.
Not just as friends asking their friends they haven't. They may have mutually met the University's requirements for alternative courses and the University has allowed the course change, and coincidentally the students knew each other and you. They most certainly didn't just decide to swap lecture schedules and get a different degree!