You know you can literally work in German-speaking countries in English-speaking jobs? Providing we stay in the EU, of course.
So yeah, you can utilise all of your time and energy on Mathematics, do Finance and work in multinational banks / financial services anywhere you like, it doesn't have to be the UK at all. You can work in Frankfurt in English-speaking job, or Zurich for that matter, and learn German there.
On the other hand, languages are always beneficial. If you could speak German on the native level you can expect many more employment opportunities.