What career should I pursue?

I'm currently a first year taking an MPharm course, and I know for a fact I won't be able to endure it for the next 3 and half years. The passion to study pharmacy just isn't there anymore. I've always enjoyed studying maths from GCSEs and through to A-Levels, and the fact that it's quite "black-and-white" makes it more enjoyable for me.
I want to switch to a course that is more maths-orientated, but I'm not too sure on the career paths after getting my degree. I just need a bit of help on what degrees are out there, and how maths-heavy they are. Obviously there's the option of doing pure maths, but I got a high B for maths A-Level, and I don't know if that'll suffice.
Any guidance is appreciated

A degree in any subject usually allows you to go into any job that doesn't require a degree (600+), a job that requires a degree in any subject, or jobs specifically aligns with the degree.
Maths degrees generally are legally required for any specific career, but there are often mathematical careers that strongly prefer maths graduates or closely aligned subjects.
The most mathematical careers (otherwise you won't require anything close to GCSE level maths) tend to include:

Maths degrees tend to focus on number patterns and math theory i.e. you would be spending most of your time doing proofs. If this isn't something you enjoy doing, I recommend revising your options. If you are more into the applications of maths like most people are, then your options are more focused in one of the following:

See: https://www.mathscareers.org.uk/degree-courses-a-level-mathematics/
You should be eligible for all of the above degree options, except for physics and most areas of engineering. If you have done Biology, you should be able to do bioengineering. If you have done physics, you should be eligible for most areas of engineering and physics.
If you are interested in physics or physics based subjects, I strongly recommend doing the A Level with practical assessment - it's going to be cheaper than doing a foundation year and it's very mathematical subject anyway.