Re applications I think people would decide between economics and maths first, if you look at course content that may help, on universities webpages and consider what their interests are. Economics has a variety of subjects which can be studied with it maths, politics, history, current affairs, development economics of different countries, some parts of geography, statistics, finance, government policy, macroeconomics of countries and is generally a much broader subject than maths. So it's a good choice for say a student who is interested in the news, government policy, the environment, supporting poorer countries to develop, politics, recent history as well as those who want to apply maths or stats, and look at options in each course. Don't need to be interested in all those and I am just out of 7 days in hospital and 11 hour op so might not be at my clearest today.
Visiting Oxford and Cambridge and subject talks would help, or get them online initially, this is E and M https://www.ox.ac.uk/admissions/undergraduate/courses/course-listing/economics-and-management
Two other economics courses at Oxford are Economics and History very small numbers and politics, philosophy and economics. E and M was 18 to 1 for places, Cambridge Economics was about half that but they are similar ability to get into. E and M my daughter said attracts a lot of international applicants for the management part but they don't self select as much. Also Cambridge Economics is more maths based and maths entry test so people with just Maths A level and no FM tend to apply for Oxford, with FM it's either but for Cambridge almost all have FM. Daughter doesn't have FM. There's lots on the university websites. Colleges themselves admissions departments might well help especially if school has poor track record. My DD did it herself and that is ideal to encourage as they understand themselves. Oxford interviews 3 per place so 15 are rejected pre interview so it's worth seeing how you would do on other criteria, gcse score, how many 8s and 9s scored same and if state at 16 was underperformed comprehensive with negative progress 8 score they adjust for that. Its within context of school. And TSA. And predicted A levels but its first three A levels I think unless candiates equal so 3 A stars is same as 4 A stars mostly in scoring though offer rate higher to 4 A stars as they generally have higher GCSE results. There are reading lists on their too, DD didn't do any of those and don't feel you need to stick to those but gives an idea. I do work in economics though and have done for years so she's grown up with it, she didn't have Economics A level either. Both do require you to be strong at maths but if they are taking maths and FM that's fine and covered. Cambridge course has a lot of options too. Interests other than in economics weren't a big factor.