There are four exam boards that offer GCSEs and A levels in England and Wales: AQA, Pearson, OCR, and WJEC. In Northern Ireland, GCSEs and A levels are also offered by the Council for the Curriculum, Examinations and Assessment (CCEA).
Exam boards in England, Wales and Northern Ireland all use the A*-E grading scale for A level qualifications.
For GCSE qualifications, the grade scale varies across the three countries. In England, the four exam boards are all using the 9-1 grading scale for reformed GCSEs as they are rolled out. Most GCSEs in Wales and Northern Ireland use an A*-G grading scale. A full explanation is available at: https://www.gov.uk/government/publications/gcse-and-a-level-differences-in-england-wales-and-northern-ireland/statement-from-the-qualification-regulators-on-changes-to-gcses-as-and-a-levels#availability-of-reformed-gcse-as-and-a-levels-in-england-wales-and-northern-ireland.