The marginal tax rate is the tax you pay on the next pound of income you earn. In the UK as elsewhere, the more you earn, the more tax you pay. If you earn less than £12,570, you don’t pay any income tax. If you earn between £12,571 and £50,270, you pay the basic rate of 20% on your earnings above your “personal allowance” of £12,570. If you earn between £50,271 and £150,000, you pay 40% in tax. This is your marginal tax rate.
So, if you earn £12,570, you’ll pay 20% on the next pound you earn - your marginal tax rate will be 20%. Or, if you earn £50,270, you’ll pay 40% on any additional earnings. When you earn more than £100,000, you start to lose your personal allowance, at the rate of £1 for every additional £2 you earn.