As others have said only BD*S and BD*G (I assume any *G could get in) are allowed into the lounge because it is a diamond club lounge and not a business lounge.

It is somewhat bizarre as I did LBA-EDI recently and the last 3 rows in the EMB were Y and every other row was C. Y was full and there were about 8 people in C. Was the same on the way down on the EDI-LBA flight as well.

A disgrace IMO offering a C ticket on a flight from LBA with no lounge access.