I don't know the exact pathway but as a general rule of thumb you should think about these:
You're in UK, keep in mind that the pre-requisite to be a dietitian may possibly differ to other parts of the world. This is true to the extent of the name of your degree course.
Generally, you'd need to have an appropriate degree which will probably be composed of the fundamentals of science -physiology, macro/micro nutrition through to advanced topics in contemporary nutrition etc. To practice as a registered dietitian, the state will likely require you to have accreditation from relevant authorities/institutions: eg: British Dietitian Association (just an example), prior to the registration they will likely require you to have an appropriate degree, probably a Masters too in addition to a number of years of experience.
ie: Undergraduate => Masters + expereince => Registration /accreditation => 100%registered/licensed with membership + international accreditation.
This is probably the strictest means of being one of the very top dietitian. If you intend just to go to YMCA or of such sort then it may possibly limit your career in many ways.