I would look at Gateway /Foundation criteria carefully. Some don't allow you to apply if you have an A at A level.
I know I have replied on your other thread, but I would suggest looking at resits and reapplying for standard a100 next year. This will give you th widest range of choices to apply to. You will need to push for a high UCAT though
So my advice would be to get that done, do as well as you can and see where you go from there.
I don't know if you are looking to resit one or two A levels or do it at school or privately. If privately you can still ask to sit the exams at the school and see if they wil do predicteds / references as part of that. It can be difficult sometimes if you have to resit any practicals as they can be challenging to arrange. See what your school / college says.
Good luck.