Are there online/correspondence courses that allow you to do GCSEs as an international student?
As a mature student you might look at other routes to uni than GCSEs and A-levels. Check out the Open University www.open.ac.uk for a range of courses on line at pre-degree and first degree level – the OU has a flexible admissions policy, so your high school grades should not be a problem. You could go right up to PhD with the OU, or transfer credits to a course a a traditional university. They offer courses in US and Canada.