You are fucked. You need a certified as copy of original photocopy, perhaps from a past work.
Otherwise you are at the mercy of the test centre having records or even still existing.
The irony is, DVLA WILL have a backup of the database somewhere.
However they will not admit that to you nor will they access the database for mere driver enquiries.
A total scandal if you ask me. I refuse to accept that it is impractical or impossible for an organisation of that size to have NO way of accessing its data backups. Also They are lying if they say no backup exists, no government database that big is NOT backed up.