First of all, sorry to hear you didn't get the grades you wanted.
Whilst it's ultimately your decision, I've been through sixth form and had friends retaking and resitting, so perhaps I can offer some insight.
It sounds to me like you're quite keen to go on to a good uni and have the drive to improve, so perhaps taking an extra year to redo year 12 may pay itself off. Yes, it's another year, but is there really any rush? You've got plenty of time ahead of you in your life. Also so many people take gap years now you won't notice an age difference at uni if you're worried about that.
I've had a couple of friends who didn't do well enough to continue into y13 but also weren't allowed to retake at the same school because of funding/spaces/blablabla. One of them went elsewhere to do similar A levels from scratch and whilst it wasn't a smooth ride, he's now got a place at his first choice uni on a course he really wants to do- something which wasn't looking possible after his first attempt at y12. I guess the biggest thing is the upheaval of your daily life- you'll be in a new school/college, needing to settle in, make new friends etc, not knowing the quality of your teachers. Could you see yourself making this massive change? I guess another benefit of taking another year is allowing yourself more time for considering uni, ensuring you go to the right place for you. Although, at the same time, three years of A levels would be considered hellish by some! It all depends on what you truly want and who you are as a person.
If you wanted to stay at your current institute, have you considered doing AS resits at the end of y13? I had friends do this and it did bump their grade up, but it was a heck of a lot of work! Extra exams, more revision to timetable, content that you haven't had taught to you that you suddenly need to regurgitate in an exam. It's tricky, but if you really don't want to take the extra year of sixth form and move away, then resits in y13 could be an option. You will really have to prove to unis that you are going to make an effort to better your grades, which can be a task in itself.
I would say that from my personal experience, you do need to be studying subjects you love. A levels are the most independent and challenging academic pursuits you've faced so far, and having a genuine interest and passion for your subject definitely pays off in your grades. If you don't have any interest in your subject then you're much less likely to put in the extra hours to get the grades you need!
Do you have teachers you could meet with or email for guidance? Perhaps an AS teacher of yours who knows you well?
It's 2am so this is very rambled but hopefully this raises some points you could consider in your decision. Maybe make a pros and cons for all your options? It sounds juvenile but I'm terrible at decision making and I find this method useful. Hopefully this jumble is of some use to you and I wish you all the best, whichever path you decide on