Did anyone here reach graduate school after disappointing A-level or secondary-school results?

I’m a 17-year-old international A2 student following the British curriculum. I received 1 A\*, 6 As and a B at O-level, followed by As in AS Physics and Chemistry but Cs in Mathematics and Biology. I have now dropped Biology, am continuing Physics, Chemistry and Mathematics, and intend to retake Mathematics.
I’m interested in studying a field related to chemical engineering and potentially pursuing a master’s or PhD later. Because I will need substantial financial support, I’m hoping to attend an affordable public university or receive a scholarship.
I would particularly appreciate hearing from people who had disappointing secondary-school or A-level results but later:
performed much better during their bachelor’s;
entered graduate school;
received funding or scholarships;
built a career they are satisfied with.
What route did you take? When did your school results stop being important? What did you do during your bachelor’s that made you competitive for graduate study? Please be honest about the difficulties as well as what worked.