I agree with everything that has been said re if teaching to 16 is inadequate then what is two extra years going to do.
The one point that I would add is that I understand maths is a bit of a 'use it or lose it' skill. Cambridge used to not allow students to take a gap year before studying maths. That certainly matches my experience - straight A student at school, including maths (but maths was definitely something that came less naturally). I did a degree in humanities and now I am shocked and a little embarrassed by the extent to which I have forgotten basic arithmetic - percentages etc. TBH it doesn't impact my daily life often though (and of course when it does, I'm able to work around it). It cannot be compared to illiteracy at all IMO.
It also, in the current economic climate, seems another way of turning blame onto the people rather than the government and its policies. NHS is struggling because people have unhealthy lifestyles. People don't understand maths and finance and that's why they can't pay their bills.