I studied economics at LSE, as did my son. DH studied PPE at Oxford. Very, very different degrees.
OP refers to top Universities. The key "quant" ones, ie those seemingly preferred by City employers when looking for technical people, are Cambridge, Warwick, LSE, UCL and now, Imperial. Oxford, Durham and others offer equally prestigious degrees, but these tend to be less mathematical - not necessarily a bad thing.
All will be very competitive, and with few exceptions, maths will be expected. The first group will expect FM, probably at A, or the potential to achieve A. (DS studied with some who had self studied AS level FM as their schools did not offer it. The pace in first year was that they still had to catch up fast.)
Less quantitative degrees will welcome history as it should demonstrate an ability to research, evaluate and explain.
However I assume that most will want to see evidence of an interest in economics. Economics will be a new subject for many at A level, and a number will not enjoy it. Ditto at University. Both I and DS came across perfectly bright people on economics degrees who just did not think like economists and so struggled. (Not unusual. Dare I say it but too many of our current politicians seem to struggle with economics, and are then surprised when growth seems to be tanking.) There are plenty of popular economics books: The Undercover Economist, Freakonomics, Naked Economics etc, and Amazon will suggest more. It is worth looking at public lectures given by Universities: LSE is particularly good as all sorts of people pass through London. DC used to follow favourite academics on Twitter, which in turn led to others to follow. There are more history, politics and economics podcasts than you can shake a stick at. Or an EPQ.
DS took Maths, FM, History and Economics, which covered most bases. Obviously there can be variations on the theme. Languages would be useful, but given how competitive the courses are, Universities are likely to ignore language A levels, unless taken as additional subjects, if there is any hint of significant exposure (ie parent from that country) outside school.
A good mathematician interested in a broader PPE degree, might also look at LSE's four year degree.