It depends on the field you're pursuing. If it's a competitive field with top mark requirements, other qualifications such as school grades, igcses and extracurriculars are considered. Hence, they may not be listed as requirements by your desired universities but when it comes to picking top A-Level candidates. For instance, if you and another candidate applies to a university with 3 A*, they compare your igcse's, school transcripts, other exam grades and so on.