The killer between £100k and £125k is that for every £2 above £100k, you lose £1 of your personal allowance (the £12,570 income you pay no tax on).
So on £100k, you get to keep £ 68,557.40
On £101k, you get to keep £ 68,937.40
For an extra £1000 income, you only get to keep an extra £380. Ie you're paying 62% tax. That's your marginal rate of tax (the rate you pay on each extra £1)
Just below £100k :
On £98k, you get to keep £ 67,397.40
On £99k, you get to keep £ 67,977.40
Ie for that extra £1000 income, you get to keep an extra £580. Ie your marginal tax rate is 42% tax.
Above £125,140, you're past that cliff edge because you've already lost your whole personal allowance So it's back down to 'Additional Rate' tax:
On £127k, you get to keep £ 79,096.40
On £128k, you get to keep £ 79,626.40
Ie for that extra £1000 income, you get to keep an extra £530. Ie your marginal tax rate is 47% tax.
It's just £100k to £125k where you only get to keep 38p in every £1. Combine that with the loss of childcare (£2k per child for tax-free childcare, or up to £10k per child for the free nursery hours) and you can end up with less money if you earn between £100-£125k than than if you earn £99k, which is fairly insane!
So most people in that income range either put the extra in pension or another salary sacrifice scheme, or reduce work.
Once you're earning above about £150k, you probably do want to take some of your >£100k as income. Taking it unevenly means that eg you pay 62% cliff edge rate on £25k and 47% additional rate on the next £25k in one single year... instead of paying 62% cliff edge rate on £25k one year and then 62% cliff edge rate on the next £25k the next year.