I am not sure it is consistent across all departments, but my UK Psychology department offers an MSc/Diploma in Psychology that can result in an MSc, a diploma, or a certificate. The course is designed to allow students who currently have a degree that does not meet the requirements for accreditation by the British Psychology Society, to get an accredited degree in one year of study. Many BSc degrees in Psychology are not accredited by the BPS since they lack, or don't require students to take or pass, a critical component (e.g., modules on statistics or biological aspects of Psychology).
Even though all students take the same modules, due to differences in the requirements between the university and BPS, the final degree a student gets depends on how well they did. The course offers both undergraduate and graduate modules. If you pass everything, you get an MSc. If you pass enough credits, you may qualify for a certificate from the university, but not meet the BPS requirements depending on which combination of modules you failed.