Contact the school you attended. They may have kept records of your A-level results and may be able to supply replacement certificates.
Contact the local authority for the school you attended. The local authority may have taken over the school records. You can find the contact details for your local authority on the website of the Department for Children, Schools and Families (see Resources).
Contact the examination board which awarded the A-level certificates. Either your school or your local authority should be able to give you the contact details for this organization.