It would be best to speak to the selection team. The certificates are checked and are required.
If you can't find them, have a look on the websites of the exam boards. Although you cannot obtain duplicate certificates unless you can provide proof of loss due to something like a fire (i.e. they won't replace ones that have simply been mislaid), you can get from them a "Statement of Results" which, while not a certificate as such, is alleged by the exam boards to be universally accepted in place of a certificate for the purposes of proving results.
This PDF from the AQA website (one of the exam boards) is an example. The first page explains what you need to do.