On leaving, workers are generally entitled to payment for accrued unused statutory leave. Most workers get 5.6 weeks of statutory holiday per year, capped at 28 days for a five-day week. Accrual and pay calculations vary for irregular-hours and part-year work. This calculator is a simple estimate; see GOV.UK.
Holiday accrues pro-rata through the leave year. If you leave halfway through the year having taken less than half your entitlement, the difference must be paid in your final payslip. Holiday pay is fully taxable, even when paid alongside tax-free redundancy pay. If you have taken more holiday than you accrued, your employer can only deduct the overpayment if your contract allows it.