If you're aim is to do IB in the future, then there's no requirement for you to take a quantitative degree, and frankly, nowadays recruiters don't really care about the degree you do.
Coming from an economics student whose career mentor has been an IB for 10 years, I've been told that they are trying to recruit more people from non-STEM backgrounds to improve their diversity, so you will see more people with degrees such as English and History get into IB. What matters is the transferable skills and work experience you get out of your degree. (So taking a less academically challenging degree that doesn't require you to study 24/7 will allow you more time to apply for work experience) My mentor has also mentioned that they are looking for different skillsets nowadays because they see the same type people who have the same Economics/Maths/STEM degrees that frankly isn't bringing much else to the table. That does mean when they interview you (depending on the recruiter) they will tailor the technical questions for what you studied, and focus more on your motivations rather than technical knowledge which they'd ask for being with a higher quantitative background.
Also side note, I didn't do A-level FM and still got into Econ at Durham - however there's a reason why they prefer people to have FM and that's because the maths modules you take, even in your first year, is primarily FM content! Yes, you could learn it at uni but it definitely sets you back if you don't have that knowledge when you start, and I've not been able to get a true fresher uni experience as I've had to learn pretty much the majority of FM independently in less than 6 months.
If I were you, aim for the top uni but if you think that FM will set you back (especially if you find that you need to practive current A-level maths almost everyday to keep up) then don't focus too much on just applying for econ- definitely keep your options open. So a less quantitative course will definitely not put you at a disadvantage. You could do one of the best degrees in the world, but if you don't know how to transfer or showcase the skills you got from your degree, then you'd be at a disadvantage. (Also considering that there are thousands of other econ graduates aiming for IB so you definitely need to develop something that makes you stand out if you took this route, hence why non-STEM degrees are currently being targeted)
I'd also look into IB/Finance related degree apprenticeships. They are much more competitive, and that's for a good reason. They are a much 'easier' route into finance and they are mainly looking for your drive/passion, rather than technical knowledge, but its definitely worth a try to apply as this can get you into the industry much faster and help you climb that career ladder.