Yes they are.
It is unlawful to ask them to waive this right by signing something to say otherwise.
Whilst the worker is not obliged to accept the work that you offer to them, the worker is still entitled to their statutory holiday entitlement which is 5.6 weeks of their normal weekly hours.
How are their normal weekly hours calculated?
By taking an average of the last 12 normal weeks worked.
Normal means those weeks that do not include periods of holiday or sickness absence.
Their weekly pay is then based on their normal basic hourly rate x their average hours worked.