Mike:
Public holidays have no significance whatsoever in employment law. The statutory minimum holiday entitlement (for someone who works 5 days per week) is 28 days (and NOT '20 days plus public holidays'). The employer is free to choose when those holidays are taken and is not obliged to take any notice whatsoever of when public holidays occur. (If, for example, an employee normally works on the day of the week when Christmas Day occurs, his employer has every right to designate that day as a normal working day, paid at normal time, and to discipline the employee if he fails to attend).
Thus having an extra public holiday added to the year adds nothing at all to an employee's statutory holiday entitlement. If (as illustrated in my post above) the employer decides to close his business on that day, it may well disadvantage employees by reducing their options as to when they can take days off.
Chris