Income tax is assessed on slices of taxable income at varying rates.
"Taxable income" is, broadly, all income, grossed up when appropriate, less reliefs, allowances and other deductions.
For 2015/16, dividends otherwise taxable at the 20% basic rate were taxed at the 10% dividend ordinary rate and dividends otherwise taxable at the 40% higher rate were taxable at the 32.5% dividend upper rate. Dividends otherwise taxable at the 45% additional higher rate were taxable at the 37.5% dividend additional rate.