Be pressurised into teaching if you do not want to. Postgraduates have no contractual obligation to teach. Made up of student representatives from educational institutions with postgraduate students, the NPC aims to promote the interests of postgraduates studying in the UK, while remaining politically non-aligned. Their website provides guidelines for the employment of postgraduate students as teachers, which may prove useful when negotiating with your employer. They offer a years free membership to postgraduate students.
Postgraduates have accesses to their universit's student union, as do any other students.
The NUS have recently extended their support for postgraduate students by updating all local branches on the issues which might arise in connection with postgraduate teachers and how they may be supported.