Since, you believe that Studies Of Religion I would not be counted towards your UAI, but in your school it is a compulsory course, I would recommend you just work a little bit on it. A band 2 is the MINIMUM BAND expected of you and is the band where you would get satisfactory in. Just at least manage a band 2 and you'll be fine.
Anyway, some of the eligibility rules for the Higher School Certificate are:
*sit for, and make a genuine attempt at, the required Higher School Certificate examinations.
*To be eligible for the award of the Higher School Certificate you must satisfactorily complete at least 12 units in you Preliminary study pattern and at least 10 units in your HSC study pattern.
Also, a quote from the UAC website about eligibility for a UAI:
*To be eligible for a UAI you must satisfactorily complete at least 10 units (including at least two units of English) of UAI courses.
I don't know if Studies of Religion I was included in your 12 units of Preliminary study because you have to "satisfactorily complete at least 12 units", but at least try and manage a pass for it. If you do, it would look better on your certificate than a fail, although either way, you would probably get your certificate anyway.