Realistic to earn a high-ish salary with a chemistry degree from a uni like Warwick?

I'm having second thoughts about having applied for chemistry before I start the course soon and I'm thinking I should have applied for something like chemical engineering from the start since that seems to have a considerably higher salary generally, though that would have required me taking maths A-level so that may have been difficult. Is it worth taking A-level maths in a gap year just to be able to apply for something like chemical engineering or are the earning opportunities for chemistry not as bad as I think?

If you go for Investment Banking or Commercial Law as a career then you will be sorted financially!!

It depends a lot.
Warwick has listed a number of companies and sectors their chemistry graduates often end up working for:
https://warwick.ac.uk/study/undergraduate/courses/mchem-chemistry (look under the “careers” section).
It does rather imply that access to the finance sector (which is pretty lucrative) is possible, but I couldn’t give you any precise figures.
Also do bear in mind that your starting salary regardless of the job you end up in probably won’t be great. Once you have several years of experience in the sector, you may well get a considerable raise.
Edit: also I absolutely love the username lol

True, but I've heard it's ridiculously difficult (correct me if I'm wrong) to get into investment banking so not sure how realistic of an option that is honestly. I'm not so sure about commercial law though, would I need to take some sort of law conversion course I'm guessing?

There are a lot of smaller investment banks as well as the big ones. Some graduates actually train as an accountant first with one of the big four accountancy firms, and go into investment banking a few years later.