Your bonus is taxed as income, added to your salary and taxed at your marginal rate. Find your salary and bonus combination to see exactly what you keep.
In the UK, bonuses are taxed as income in the same way as your regular salary. Your employer adds the bonus to your earnings for the pay period and applies PAYE, Income Tax and National Insurance, at your marginal rate. There is no separate "bonus tax rate"; it depends entirely on which tax band the bonus pushes you into.
If your salary is £40,000 and you receive a £5,000 bonus, your total income becomes £45,000, still within the basic rate band. You'll pay 20% Income Tax and 8% NI on the bonus, keeping approximately £3,600. If your salary were £48,000, part of the bonus would be taxed at 40%, reducing your take-home.
Tip: If your bonus pushes you over £100,000, you begin losing your personal allowance (£2 for every £3 over), creating an effective 60% marginal rate between £100,000 and £125,140. See the £100k tax trap guide. Salary sacrifice into a pension is the standard fix.