In the UK (and in Hong Kong, where some schools still follow British models), there are so-called conversion courses for college graduates to convert their majors (e.g. from business to psychology) by doing a program that provide just the minimum required coursework for the target discipline. The coursework alone does not amount to a second degree (so, a student does not end up with a BA, for example, but a cert. or diploma).

These programs have different academic weight or demands for required coursework. They may be arranged (in ascending order) like this:

graduate certificate (GC) -- covers some UG (undergraduate) courses for a major

graduate diploma (GD) -- usually covers all UG courses required for a major

pre-masters program -- for preparation for a masters course (when a person's previous UG work is found not sufficient)

postgraduate diploma (PGD) -- covers some masters/PhD coursework for a major but eventually the candidate may have chosen not to finished all coursework (esp. for masters degree)

So, in terms of workload, the order is like this:

GC < GD < PGD

The pre-master is a fairly loose term, which can be part of a GC or GD.

FYI
http://www.gre.ac.uk/international/pre-session_english_cours...
Pre-Master's Programme
For international students who wish to study for a Master's degree in Business, Computing or Engineering at the University of Greenwich but whose academic qualifications and language level may not quite satisfy the requirements for direct entry onto a postgraduate programme.

Further details of the International Pre-Master's Programme


http://prospectus.shu.ac.uk/op_pglookup1.cfm?id_num=419
Graduate Diploma Psychology
Full-time and part-time

--------------------------------------------------------------------------------

This course is for non-psychology graduates who have some previous study in psychology, including joint psychology degrees not accredited by the British Psychological Society (BPS).