I suggest you to call the university direct. My son had the similar experience last year. He sent emails to check the status of the CAS but no one answered his emails. Finally, he called the university office, and the staff checked the status instantly and sorted out the problem which hindered the issuance of the CAS. He got the CAS two days after the phone call.
BNO can only treats as a type of passport and they won't treat you as "English" (英國人). You still need a student VISA!! You can get in UK but does not mean that you can study in UK as a student. Pls check with University that you can start your course WITHOUT a VISA or not first.
1 thing you can do is delay your flight ticket & arrive your University when VISA is ready!