Want to re-apply, please help.

I'm currently studying Physics at The University of Warwick. I have been on my course for a week and I'm certain I want to leave. My A-Level grades are A*AB in Maths, Chemistry and Physics.I want to change my course re-apply through UCAS. I want to study Economics hopefully at one of the following institutions, UCL, Durham, Nottingham, Bristol and Bath. During my time out I plan to take up Further maths. I have to hopes of going into IB/Consultancy. I have no idea how to begin the process or if its worth it. Would it be worth leaving and do I stand a chance in getting offers from the universities stated above. How would I begin the leaving process and how would my student finance be affected.
Would I be disadvantaged in my application?

With A*AB you don't meet the standard requirements for UCL, Durham, Nottingham, Bristol and Bath which is A*AA at most unis apart from Nottingham where its A*AA-AAA, and no A*AB is not equivalent to AAA. Although Bristol do give out AAB offers for those who are underprivileged if you meet the criteria though. So it would be very foolish to quit your current course to apply for the uni's you've mentioned - one or two maybe as high risk choices, but most should be unis you meet or exceed the requirements for.

What if I sat Further Maths and Economics during my year out? Would this be considered

You'd need to email admissions and ask if they would consider A-Levels taken in more than 2 years

Despite being to completely new A-Levels? Ok. Would me have gone to University or a week impact my application and put me at a disadvantage?

Email them up tbh!If you do get A*s/A's in Further Maths and Economics,you can try to get into UCL etc. via adjustment(this again depends on places left)

Hi is it possible to re-apply whilst at university? Would I be disadvantaged.

Email them up!Are you doing the first year of uni or are you dropping out?

As of right now I'm currently on the course, I'm considering leaving.

There were extenuating circumstances during my exams.

Look OP.Mention your extenuating circumstances in your reference.
You've got great grades.They can be better only if you work hard.Email those unis(I am telling you this is useful)
Get A*s/A's in your new subjects!
Tell the unis your grades from GCSEs to A levels and your circumstances