The CAA do not provide any such certificate. The only proof of passing the exams is the results letters that are issued after exam exam. If you require a replacement letter the CAA will be happy to provide one, but they will make a charge for doing so. So candidates should retain these for future reference.
Some years ago the CAA Standards Document for schools providing ATPL theoretical knowledge courses included a course completion certificate. But Standard Documents are only advisory and most FTOs decided that providing a certificate was too much trouble. So the idea died.