Zakat On Benefits
You can choose to pay zakat. But if the money is solely used for her as a separate person and received on her behalf, then separate calculation of that amount can also be done on her name. As for nisab on income/salary/production, please follow the tax exemption point/slab for that particular source of income in UK tax law. Whatever threshold is considered taxable, the same amount should be considered nisab for zakat purposes. Nisab is the amount below which zakat is exempt. It is true that if the tax paid is more than zakat amount, then there will be no zakat liability. Infact, excess tax paid can be carried forward to the next year to offset against next year’s zakat amount. Zakat calculation still needs to be done for both zakat on production/income and zakat on wealth , for which percentages and Nisab thresholds are different.