International student in the UK

Hi! I'm planning to move to UK from Sri Lanka in a few months and me and my family have been wondering how to start Sixth Form as a 17 year old. Here in Sri Lanka we go to school till 19 and according to my estimations I am also a little back from the UK standards of syllabus. Will I be able to catch up?

It is also a question that due to financial disabilities if we will be able to afford school after I'm 18 as free education is only applied until then.

Sincerely would love to have a little bit of help in this matter

I would consider liaising with the admissions staff of the 6th form college that you wish to attend. As you are from outside of the UK and done qualifications that don't quite fit the UK curriculum (unless you're telling me you did iGCSEs), then you would need to get advice on your particular circumstances. In terms of enrollment, you usually submit your results along with a personal statement to the particular college that you wish to attend, but you should contact admissions regarding yoru circumstances first.
6th forms don't strictly require you to leave the moment you leave 18; there's usually some flexibility up until the academic year when you turn 19.

I don't know what you mean't by financial disabilities. Do you have financial difficulties or disabilities? If you mean the former, then there are usually bursaries and scholarships for people from low income backgrounds. If you have disabilities, there are usually support for students how have certain challenges (e.g. wheelchair access, transcribers during exams, IT software to aid access). You can usually reach out to the college or Student Finance to check, but you would need to be specific.