I had an interview and offer from Manc last year, and my academic profile was worse than yours, and I had a UKCAT of 662.5. As theatrical said, NI students are welcome in any UK uni, and it is most likely that your PS and ref didn't pass.
I'm not sure about your ECs, but Manchester was one of the first (if not the first.. can't quite remember) unis to implement a full PBL curriculum, so teamwork would be a very high priority for them. If your PS didn't show this clearly enough - teamwork, communication, respect for colleagues etc - this might be a reason for them to reject you.