Litch in the same position. I’m gonna sit the exams but I’m sure I won’t get the grades I’m capable of. From what I’ve researched already you can resit year 13 if your school/college allows you to do so. Or you can apply as a private candidate but then you are responsible for applying to sit the exams. I think the deadline is 6 months before the exam dates (something like that). You can also go to an independent college but they are stupidly pricey but help with exam practice. If a private candidate you can revise independently for the year or do online lessons, either way you have to pay for the exams.
I’m still doing research but this is what I’ve got so far hope it helps x