I don't think that Lucy Kellaway has stayed as a full time maths teacher. According to a TES interview in 2018 she now teaches part time and only teaches business studies / economics, which is very different to teaching a core subject full time. So I take Lucy Kellaway with a pinch of salt...
From TES website-
"Exclusive: Teaching full-time 'unendurably hard', says Lucy Kellaway
By Will Hazell on 27 November 2018
Now Teach founder and former Financial Times journalist describes her year teaching maths as 'hell'
Schools are stuck “in the dark ages” when it comes to part-time working, the co-founder of Now Teach has said.
Lucy Kellaway, who started Now Teach to help older professionals in other occupations switch into teaching, said her own decision to go part-time had made the difference between teaching being “unendurably hard work” and “completely manageable”.
Ms Kellaway worked as a Financial Times journalist for 30 years before founding Now Teach and retraining as a maths teacher in a London state school last year.
During her training year, she worked full-time, but since becoming a newly qualified teacher in September, she has switched to teaching business studies and economics and has moved to three days a week.
Ms Kellaway told Tes she had always planned to work part-time after her first year, and that this move, combined with her change in subject, had “transformed everything”.
“It was hell the first year but, astonishingly, now I am really loving it,” she said.
“The two things that have transformed everything for me is that I’m teaching something that I adore, and just as important, I’m [working] three days a week.
“It seems to me that working three days is the difference between teaching being just unendurably hard work, and it being – it’s not easy – it’s just completely manageable.
“It just makes me so sad that every teacher can’t do three days a week.” " etc etc etc
And the reason every teacher can't do three days a week is it pays 3/5 salary and most people don't have the financial cushion of having been a finance journalist/ editor for years... I assume she has a substantial investment portfolio.