You should really get experience, shadowing a dietitian or optometrist, or volunteering in a dietetic or optometry department before joining any of those courses.
I'm due to qualify soon, placement ends in 5 weeks. I'm studying dietetics, post grad, masters. If I got proper experience beforehand in the dietetic department, I wouldn't have joined this course. I'm not sure it's the career for me, but I'm hoping I can still qualify just so these 2 years are not a waist. If dietetics is what you really want to do, do the undergrad, you get more support since its 3/4 years depending on the university.
My course is the first cohort at my university, so I guess lack of support and organisation is expected. Go to a univeristuy institute that has a well established course, - mean they've been running it for years because it's likely gonna help having previous students there to help with practice placement and academic work.
All I'm saying is go to alot of open days, don't worry about the rank of the university because each course might be different quality standards. Not sure about optometrists, but dietetic course have to be accredited by HCPC and BDA. So all dietetic courses have relatively same standards but how they deliver it is key and that's what you need to find out when you go to open days. Are classroom lessons more practical, or theory. How much time do you spend on each topic. Will fhied method of teaching match with your leanring style? My experience was whistle stop tours for every topic and alot of independent working, more so than my undergrad degree which I thought was not possible.
Ask yourself are you just interested or actually passionate about dieitics or optomotry? Because there is a difference in how it will affect your educational experience. Passion will get you through it. Interest you will be destroyed, physically and mentally.
Sorry this wasn't more upbeat, I just thought I'd be honest. Hope this helps.