No worries!
I remember our school highly stressed that you should accept first round as the chance of second round is considerably lower. However if I do decide to wait, i may have a chance for a spot if they still have seats available?
Yup, that info is true. Chances in Round 2 will be much lower than Round 1 and in any round afterwards. Definitely accept the offer if you're cool with doing it for a little while (which you seem to be) and ensure Preference #1 is the BCompSci as I stated above. You'll still be eligible for any higher preferences, even if you accept an offer in Round 1, i.e. if BIT is Preference #2, you can still get the BCompSci at Preference #1. This includes if you switch preferences before Round 2. If they have any spots in the BCompsSci available and you've got everything else (i.e. you fulfil all the selection criteria, yadda yadda), they might just grant an offer for the BCompSci to you in Round 2.
If you reject the offer now, you might risk the chance of not having any offers made whatsoever, which is a sticky situation that you
do not want to be in. (Unless you can't stand studying IT for a little while but can stand Comp Sci for some reason, please accept the offer.)