The first £12,570 you earn is your Personal Allowance and isn't taxed. Income Tax is then 20% up to £50,270, 40% up to £125,140 and 45% above that. National Insurance is separate: 8% on earnings between £12,570 and £50,270, and 2% on everything above.
Above £100,000 the Personal Allowance shrinks by £1 for every £2 you earn, and it's gone by £125,140. Losing the allowance while paying 40% on the same income makes the effective rate in that range 60%, or 62% with National Insurance, which is why a pension contribution that brings your income back under £100,000 saves so much.
On your payslip, your employer spreads all of this across the year through PAYE and your tax code, 1257L for most people. Scotland has its own Income Tax bands, so use the Scottish calculator if you live there.