If you have lost your certificates, you will have to apply directly to each of the exam boards.
The awarding bodies will not issue replacement certificates (unless due to fire, flood or theft) but will instead issue a Certifying Statement of Results which employers/organisations should accept. A fee of approximately £43 per exam board will apply.
Links to the exam boards are as follows:
AQA
http://www.aqa.org.uk/contact-us/past-results-and-lost-certificates
Edexcel
http://qualifications.pearson.com/en/support/Services/replacement-certificates.html
OCR
http://www.ocr.org.uk/ocr-for/learners-and-parents/lost-or-incorrect-certificates/
WJEC
