I do wonder if we undermine the idea that "basic maths is a core skill of being an adult" by requiring specialist teachers to teach it.
If it's something that every adult can and should be able to do (e.g. split a bill) then demonstrate by having every teacher do basic sums in their lesson as if they're no big deal.
If a history teacher can't ask a pupil how many years were between Battle of Hastings and Magna Carta (and be able to show them how to work it out if they struggle) then you reinforce the idea that only people good at maths need do maths...
Sadly I suspect that battle is lost as our kids are already told that spelling doesn't need to be corrected outside English lessons, so presumably numeracy isn't required outside maths.
I remember that the GCSE mock exams at my school for some subject were marked out of ninety, as the teacher for that subject could not work out a percentage. Doh! Your comment reflects my thought that Mathematics could be demonstrated practically.
When I was at school you were pulled up for writing 'oxagen' or "mathamatics'.