What is the job market like for someone aspiring to become a mathematician (uk)?
You're right that the job market is academia is very competitive, across all subjects, not just maths. While raw talent is important to be able to succeed (and by &quot;succeed&quot; I mean get a permanent job in academia -- your definition of success might be different), your publication record is generally more important. The quality and volume of publications you write depends on many things: your own hard work, the work of your collaborators, the guidance you get from your supervisor (having a distant or disengaged supervisor can seriously hamper your progress during your PhD unless you're extremely good at self-motivating and learning), the tractability of the problem you're working on, etc etc.
All this to say that, once you start your PhD, the classification of your undergraduate degree doesn't matter. Working on interesting publishable problems with a wide range of people (ideally collaborate with academics outside your own university if you can) will give you the best chance at success. It's also important to advertise your work as much as possible, and don't be afraid to relentlessly self-promote; go to conferences, talk about your work to everyone you meet, convince them that your problem is interesting and your solutions even more so.
I think you have nothing to lose by doing an MSc (apart from a year's potential earnings). Try to choose a course with a research or dissertation component. This will give you a chance to explore what doing research is really like, and will help you decide if doing a PhD is right for you. Your Master's supervisor or tutor would also be able to advise you about that, especially if you tell them early on that's what you're interested in.