All workers are entitled to holiday pay.
That includes employees, agency workers, casual workers, apprentices and zero-hours workers.
In fact, even if someone is classed as self-employed, they might still be a worker. If they do the work themselves and there’s nobody else to do it (no right of substitution), they’re a worker, and if they aren’t doing the work as part of their own business, they’re definitely a worker.
And remember that holiday means “paid time off” not just time off.
Part time workers are not automatically “on holiday” when they are not working.
Part time staff have a right to the paid holiday, pro rata their actual worked hours.
Not just their contracted hours.