Have you checked your email (esp. your junk mail) to see if they sent you an email asking for predictions?
This is what most Unis would do. If you didnt reply to that email they of course they will eventaully reject you.
I would have thought unis were used to this from international students.
If you had read the UCAS infromation properly, then you would have known about the need for predicted grades from your referee. I dont like the shifting it onto the referee as its your responsibiliyu to do the research and understand the process fully. Just do as Doones said and see if you can rescue it, especially if you really wanted Manchester.
contact the uni via email and provide a copy of ur predicted grade and ask if they can do anything about it also your agents a idiot. However worst case sceniory you can still go through clearing