Can my IGCSE teacher write my reference for UCAS?

Certainly, having your IGCSE English teacher write your reference for UCAS seems like a feasible option given your unique circumstances. It's evident that you've built a significant rapport with your IGCSE teacher over the years, and her familiarity with your academic journey and personal growth could add depth to your UCAS reference.

Your situation of joining a different school for your AS and A levels, coupled with the challenge of self-studying, presents a distinctive narrative that your IGCSE teacher is likely well-equipped to convey. Her perspective on your dedication, resilience, and growth since the 7th grade can offer valuable insights to the admissions committee.

If you are school, your school will write your UCAS reference.

Only if you are applying through UCAS as an independent/private applicant will you be able to nominate a specific person to write your reference - and you need to ask them for their permission before you do this as they may not be willing to comment on your A level studies or provide predicted grades as they are no longer teaching you.