1) You will need a 3-year degree in Psychology that the British Psychological Society accredits. This gives you a GBC (Graduate Basis for Chartered Membership of BPS). You usually need to achieve the minimum of an upper second class honours degree to go on to postgraduate study.
2) You will need to obtain relevant work experience.
3) You will need a PhD in the area of Developmental Psychology.
GBC
Graduate Basis for Chartered Membership of BPS - Basic requirement for postgraduate study in Psychology