It doesn't need to be in their contract. Your employer can decide when your holidays are. It's well within your rights to say they take 2 weeks over Christmas. You can even tell them to take holidays if your vans off the road I similar with short notice
You cant force your employee to use a holiday entitlement if you van is of the road you can ask but they don't have to
agree.
A employee contract sets out the rules that both the employer and employee agree to it doesn't work one way.
If you agree to employ for a set amount of hours per week then the employee is entitled to be paid for them hours
as long as they turn up for work as agreed.
If you don't have enough work then you can send them home but you must pay them as agreed.
Holidays work the same you can set out what ever system you want either set days or floaters but you have to
stick to it.
To change your holiday contract you have to give a certain amount of notice to all employees involved.