If you have lost or misplaced your original GCSE/O-level certificates you have several options: request a replacement verified statement of results, take an equivalency test or retake a GCSE.
Before you retake a qualification, contact the school where you took the exams and ask if they have a record of your result or can tell you which exam board you took the exam with.
On the gov.uk website you can find information on how to contact the relevant exam board to see if they are able to reprint your results. The page also has details of who to contact if your exam board no longer exists.