You will be liable for your tuition fee loan, as you are still enrolled on current course and it has been running for more than 90 days. However you do not have to start repayments until the April after you leave the course and only if earn more than £21000, if you are subject to the September 2012 fee regime. So it would not be wise to officially leave before April 2013. If you don't stay enrolled until the summer term has started, you may not be eligible to recieve the summer maintenance loan payment if you are eligible for that.
As you are planning to start a new course in September, your loan repayments (tuition and maintenance for your current and future course) should be deferred until you complete the course you start in September. You may need to complete some paperwork with the student loan company to confirm that you are not eligible to start loan repayments for the course you are leaving. So don't ignore letters for the student loan company or assume that because you get confirmation of loan for your new course that their records have been properly reconciled.
If you complete your 1st year successfully of your current course, it may act as an insurance policy if your A level retakes do not meet your needs. Or you have a change of heart about what you want to do.
Staying on until at least the end of April, should trigger your final maintenance loan payment if you are eligible. Otherwise you may have to rely on parents finanncially between leaving your current course and starting a new course. You are unlikely to be eligible for job seekers allowance income based as you will not be available for work due to your A level studies.
Ultimately it is up to you to decide what you want to do. But if you need the support of your parents to achieve your goals, it would be wise to take a little time to explain your plans in detail to them and try and get them into a fully supportive mode. Hopefully, once they understand your plans, they might be reassured that you have realistic plan in place.