I think the first thing you should do is look at your GCSE's, were there any specific subjects that you thoroughly enjoyed and/or got really good grades at it? Although there is a real big gap between GCSE's and A-levels, and I assume you realised that, the work load would be too much if you move onto Year 13. I suggest you want to build yourself back up so retake Year 12 with perhaps a different set of subjects?
I personally retook the year by going back to the sixth form attached to my school, college is a really independent atmosphere so you'll have to really push through if that is your only choice.
Also check the exam boards when looking at other places to do A levels. If worst comes to worst a good idea is to take a combination of Btec and A levels as well. I know some people who got into courses that require higher grades for A levels but they have a combination of both with lower A level result.
Best of wishes, it is tough but remember you'll be retaking the year for yourself and not for anybody else. Be totally honest with yourself about your options and don't spread yourself too thin three subjects suffice!