All workers (and therefore all employees) by law have a right to a minimum of 5.6 weeks paid annual leave. So, someone working five days a week would be entitled to 5.6 x 5, a total of 28 days. The number of days of annual leave may include the usual 8 public or bank holidays in England and Wales. Someone working two days a week would be entitled to 5.6 x 2, a total of 11.2 (which can be rounded up to 11.5) days a year. There are 9 public holidays each year in Scotland and 10 in Northern Ireland, which are included in the Statutory Entitlement of 28 days for somebody working 5 days per week and pro rata for those working less.
There is no statutory right to public holidays, so an additional public holiday does not increase any entitlement to holiday under the Working Time Regulations. Whether or not an employee will benefit from the extra public holiday will depend on the wording of their contract.
As an employer you may, of course, offer more than the statutory minimum. However the exact number of days can also be dependent on specified working hours or working pattern.
See www.hmrc.gov.uk