Some students may feel disappointed that they haven’t been able to sit their exams. If they do not believe the correct process has been followed in their case they will be able to appeal on that basis. In addition, if they do not feel their calculated grade reflects their performance, they will have the opportunity to sit an exam at the earliest reasonable opportunity, once schools are open again. Students will also have the option to sit their exams in summer 2021.
https://www.gov.uk/government/news/further-details-on-exams-and-grades-announced
source – Education Secretary Gavin Williamson 20/03/20
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UK Government advice – Link – coronavirus covid-19 cancellation of GCSE,AS and A-levels in 2020