(Amended) Restarting year 12 and the effect on university applications.

I am currently in year 12 studying Maths, Further Maths and Physics.
My general motivations for wanting to resit are that I am currently in a situation where I have had no teacher for a month (previously), and am about to be in the same situation again. As well as wanting to pick up another subject - Computer Science - , I'm not particularly enjoying the environment (students) and lesson structure of where I am. I have been offered a place at a good sixth form near me already.
I am mostly decided upon applying for a mathematics or a computer science degree at a Russel group university. However, after reading through a few threads and university websites I'm still unsure of how large of an effect resitting year 12 (No AS exams will be sat) may have. For example, UCL defines resits as attempting to improve a grade you hold an award for, which (I hope?) does not include my situation. However, I'm also aware that applying to universities like UCL may be far fetched if I do retake the year.
I'm still not fully decided - my mock results will certainly help after I have taken them - but would appreciate any information/experiences people can provide.
Thanks

I have zero knowledge of computer science or maths but I can tell you a little about resits.
I was in a similar(ish) position to you, but it was during secondary school. My school wanted me to resit Year 10 due to my low attendance, to give me a better chance at GCSEs, but I declined, since I wanted to move on with my life. In terms of how that could have affected university, basically it wouldn't have. Generally, when defining what counts as a resit, universities will say that resitting an actual qualification is what constitutes a resit, and from there, they decide whether they accept this. If you were to sit AS examinations at the end of the year and then transfer to another school that also requires AS examinations whilst doing the same subjects, they would consider that a resit. If you aren't doing AS qualifications at either your current school or your new school, then that is not a resit. Universities do not care if you've restarted a year of school, as long as you haven't sat any qualifications that you will be sitting again. UCL would not count your situation as attempting to improve your grade, because you would not hold an award for it at the end of the year and the vast majority of Russell Group universities would not care. I cannot speak for Oxbridge, but I do know that there are people who have taken gap years to resit certain A levels and have gotten into Oxbridge, so I believe this rule would apply there as well. If you are unhappy in your current school, regardless of your results, I would recommend making the switch if you feel you would be happier there.
Hope everything goes well for you!