Trying to understand UK tax brackets for salary above 150k

• £0–£50,270 → 20%

No - it's £0 - £37,700 -> 20%

The basic rate band is technically a certain amount of money that falls within it, not a ceiling, so with the personal allowance disappearing both the bottom and the top of the band move.

• £50,271–£125,140 → 40%
• £125,140+ → 45%

The £125,140 threshold is correct, but the starting point of the 40% band is £37,701.

So your calculation becomes

0.2 × £37,700 + 0.4 × £87,440 + 0.45 × (income − £125,140)

Note that this is only applicable for people earning >= £125,140, for whom the personal allowance (i.e. 0% band) is completely withdrawn. This was clear from the premise of your question but is perhaps not obvious for all readers.

Reference

Thanks to @Ganesh's link.

The actual brackets for 2023 or 2024 are:

• Up to £37,700: 20%
• £37,701 to £125,140: 40%
• Over £125,141: 45%

Therefore the formula is the sum of:

• 20% × £37,700 = £7,540
• 40% × (£125,140 - £37,700) = £34,976
• 45% × (income - £125,140)

Or, simplified: £42,516 + 45% × (income - £125,140).

For £150,000 this amounts to £53,703 owed in taxes.

There is a personal allowance on earnings from interest rates, for folks ending in the basic & higher brackets, but it disappears for folks ending in the additional (£125,141+) bracket, so it's not applicable to someone earning over £150,000.