Did you not read the rest of my post? The little bit about e-borders? That's the important bit. Since non UK nationals can get into N.I without having to show a passport, then to leave N.I, you need to show your passport or I.D. This is different compared to any other internal EU flight.
One thing more. You only need one non-designated airport before you have to tell SB, not 2. So if you are flying into BHD from a non-designated airfield, then you need to give notice.