It's not too difficult.
Any income you earn over your personal allowance (normally £12,570) up to £50,270 is taxed at 20%
Any income you earn over £50,270 up to £150,000 is taxed at 40%
Any income you earn over £150,000 is taxed at 45%.
And it's only income in those bands that get taxed at those rates. So say your salary is £50,000, anything above your personal allowance is taxed at 20%, so you'd pay 20% on £37,430. If you then get a pay increase to £52k, only earnings above £50,270 is taxed at 40%, so only £1,730 of your income is taxed at 40%, the rest is still taxed at 20%.
It's a common misconception that if you get a pay rise into the higher tax bands you actually get a pay decrease, but you don't; you just take less of the increase home.