I'm a Computer Science student in the UK who is about to go into my last year and I'm looking into applying for a masters programme and I'm not sure what subject areas I can apply for. I've heard that doing a Masters in Computer Science after doing your undergraduate degree in Computer Science is essentially useless. I'm on track to hopefully graduate with a first if that effects what I can do after I graduate. I was particularly interested in pursuing something maths related.
There are many kinds of masters program you can apply for with a degree in CS, ranging from more "technical", specialized programs to non-technical ones like an MBA. Restricting the possibilities to more technical master's programs "that involves some maths" and/or advanced programming still cover a large number of potential programs, especially if you're willing to go abroad. Examples include master's in machine learning, bioinformatics, financial engineering, embedded systems, information security, data science, and scientific computing.
It'll be best if you first consider clearly which specific area you want to specialize in. If you are still uncertain about this part, it'll be good to work for a few years first. Working also serve as a kind of reality check -- you may think that you will enjoy a specialization based on your experience with a related module during your undergraduate years, but later discover that you don't really like the job scope of a typical professional in that field, or any of the potential employers in your location, or the overall long-term prospect of that particular field. In that case you can still adjust your plans before investing more time, money and effort to get a Master's in that particular specialization.