They all have great career projections. I don’t know much about medicine so the threads above should really help. I would believe that if you go into medicine, it is unlikely you'll ever find yourself unemployed as we have a shortage of doctors.
Specifically for Maths with Economics or just Maths, internships and placements while at university may make it easier to find a job post graduation but that is a few years into the future for you.
If you go to do for Maths with Econ, you won't be able to apply to Oxford, Imperial or Cambridge, as they don't offer this course unfortunately (they may in the future you never know). As you seem to be interested in these unis maybe Maths is best if you decide that medicine is not the path you want to go do.
Lastly, I would not recommend applying to some unis for Maths and others for Maths with Economics can make your personal statement harder to write and may lack focus. So keep that in mind.
Go to open days, ask questions, maybe do some work experience if fields relevant to those degrees while in year 12, all of these helped me decide what I wanted to do.
Good luck with your A Levels, hopefully some bits of this helped.