I know a bunch of people who dropped down to general in my school and their marks didn't improve, even if they said it was 'easier'. General is completely different to 2U, it's not like dropping from ext1 maths to 2U.
In terms of IT in uni, assumed knowledge /prerequisites differs from place to place. Where do you want to study? If it's assumed knowledge, yes, you can take a bridging course. But some places even have Ext1 maths as assumed knowledge..
In the end it depends on whether 2U is really going to bring your UAI down. If it does, to the point where you won't be able to get into IT, then maybe consider general... but you said you "didn't try"... why didn't you try? why don't you try from now on? it's not too late. I had a bad geography teacher who only taught for 10 minutes, poorly, and told us just to do the rest ourselves. I pretty much taught most of the course to myself, as did the class. You just have to deal with teachers like that and get over it, and not blame them for doing bad in something. Get a tutor if you can afford it, or ask if you can spend 1 on 1 time with your teacher or other teachers at times during the week.