Yes, you have to pay holiday pay for all employees irrespective of the size of your work place. Paid annual leave is an issue of health and safety. Whether you have one employee or 1,000 employees, all are entitled to 28 days holiday, pro rata, per year.
Bear in mind that this rule is not restricted to employees but covers the broader definition of workers so will also apply to anyone you have working for you on a temporary, seasonal or casual basis.
The amount of paid holiday will depend on whether or not your employee works set contractual hours for set pay or if their hours and pay vary. If they have set hours then that is how you will determine a week’s pay for calculating paid annual leave. If their hours and pay vary you will need to work out their average pay to determine this rate.