Which is fine if you don't need the income. It's not really reasonable for the government to effectively cap the income you can earn - by making it financially negative to go above £100k unless you can jump all the way to about £150k.
For a single parent on £100k in London with 2 kids in nursery, spending £31k taxed income on nursery (which includes 30 free hours and £4k tax free childcare), £24k on renting a very ordinary 2-bed suburban flat, £6.75k student loan, £2.4k zone 1-4 travelcard to get to work, she's left with precisely £360 per month for the 3 of them to live on (food, council tax, bills, clothes, necessities etc). But no help from the government, because she earns so much!
If her salary is £120k, she might like to actually take that extra £20k to live on instead of putting it into pension. But on that extra £20k she would pay £12.4k tax, £1.8k student loan repayment, and would lose £4k tax free childcare and £10k funded hours. So she can't.
£360 per month is the most the government allows her to take home per month to live on (for 3 people) after childcare, rent and commuting.
She needs to jump to £142k before she can take home a single penny more!
Our tax system is completely ludicrous.