I built Plain Sums as a set of free calculators for UK money questions: CIS deductions and refunds, Inheritance Tax and probate, gift taper, late payment interest, the VAT threshold, window cleaning price rises, stretched funded childcare hours, allotment costs and Christmas collection slots. Everything runs in your browser and nothing you type is sent anywhere. Each one links to the GOV.UK rules it uses... they give estimates, not tax advice. Some pages link to optional paid spreadsheet trackers.