They're entirely unrelated courses, so you really need to decide what you want to do long term. You could do an optometry course after a ChemEng course and still be funded, as it's an allied health profession - although this will delay you entering the work force, and allied health professions in general tend to have lower remuneration than other sectors, which may be a problem if you want to e.g. start a family, get on the property ladder etc.
The reverse is less true - you could only do a ChemE course after optometry and be funded if you do it part time. While there are some very good part time courses available in this area, they're mostly in northern Scotland, which may affect your decision making in this regard. Not everyone wants to live/study in Dundee/Aberdeen (also unsurprisingly given the location, these are more skewed to oil and gas engineering which is a shrinking niche).
Also neither course is dentistry, and accounting is completely separate to all these areas. I would strongly recommend you take a gap year to spend some more time figuring out what you want to do. It's a lot easier (and cheaper) to start off on the right course for you, than switch continually - take it from someone who did that
A gap year isn't going to make a difference in your long term earnings potential, or your attractiveness as an applicant, and can be very helpful in giving you additional "space" to figure out what you want to do - ideally, while earning some money and experiencing some element of independence, and maybe getting some relevant work experience (or qualifications - it would be a good idea to consider doing Further Maths and/or Physics if you decide to go down the ChemE route in a gap year).