Calculate UK dividend tax liability

Estimates the dividend tax a shareholder would bear on a gross dividend, given their other taxable income and the personal allowance already used against it. The other income determines which tax bands the dividend falls into: the dividend allowance is applied first, and the remainder is split across the basic, higher, and additional bands. All arithmetic uses decimal rounding to the nearer penny at each band. The result is an advisory estimate, not a binding tax record, and creates no journals. The rate and band figures applied are returned in the response.