All JAR approved Groundschools are required to issue their graduates with a Course Completion Certficate, which must be signed by the CGI or Head of training.
For details go to the Flight Crew Licensing section of the CAA website and read Standards Document 33.
Unfortunately not all schools actually comply with this requirement.
If you have completed the course and passed all of the exams, you are entitled to receive a certificate.
If your school does not issue one you should refer them to the Standards Document.