That's a bit of a simplification.

From £9.5k to £50k you pay 12% national insurance in addition to income tax, so it's really 32% not 20% tax.

Over £50k you pay 2% national insurance.

Your employer will also be paying 13.8% national insurance.

Once your income exceeds £100k you start losing your personal allowance at a rate of £1 for every £2 over £100k, so you have no personal allowance after earning over £125k. This means that your effective marginal tax rate between £100k & £125k is about 60%