From experince working in the NHS managing both professional and support staff, can i point out that if the Op is interested in the apprenticeship route to being a Dietician and not an assistant or a dietetic practitioner, then its worth confirming that he/she will be able to get a place at Uni based on the apprenticeship scheme, and that the NHS Trust that he/she works for in happy to sponsor him/her to work through the various NVQ or equivalent qualifications that will be required. The fact that it is theoretically possible doesn't mean that it is possible in practice.
The employer may not want more Dieticians and therefore may not want to invest in training aimed at that outcome, also according to the British Dietetic Association website, there are no part time Dietetics degree courses in the UK so at some point OP, you'll need to become a full time student.