Once you have a degree, it is possible to take a masters degree (MSc) or postgraduate diploma, which typically take 12-18 months to complete, or to study for a doctorate (PhD), which takes a minimum of 3 years.
A masters degree in nutrition is a means by which you can change careers if your BSc is in a subject other than nutrition, or by which you can specialise, e.g. by taking a postgraduate course in public health nutrition or sports science. Some masters degrees in nutrition have been accredited by the Association for Nutrition (AfN) and can be viewed here.
A list of universities that offer postgraduate diploma courses in Dietetics can be viewed here.
To undertake a PhD, it is necessary to have been awarded a very good BSc (first class or upper second (2i) class honours) or to have a MSc in the subject, together with funding that is typically in the form of a research grant.