A Maths degree usually covers three main areas: maths, statistics, and operational research. Maths will look at the theories and proofs behind complex problems in mathematics and is interested in numbers, shapes, and space. Statistics is often driven by real world problems which rely on examining data to make predictions - this can be useful for deciding the efficacy of new healthcare treatments or business decisions. Operational relations looks at decision making procedures such as game theory and can be useful in the field of economics.
People in school to speak to: ANYONE IN THE MATHS DEPARTMENT :)
According to UCAS, the average offer for a Maths course at UK universities is AAB - HOWEVER - this will differ depending on the university you apply to.
UCAS: https://www.ucas.com/explore/subjects/mathematics-and-statistics
USEFUL SUBJECTS TO HAVE STUDIED: Maths/Further Maths/Physics/Economics
University Subject Guide: https://www.thecompleteuniversityguide.co.uk/subject-guide/mathematics
Careers: https://www.ucas.com/explore/subjects/mathematics-and-statistics#careers-where-it-can-take-you-