Employers CAN dictate when folk take their holidays. This is what it says on the ACAS website-
"What restrictions may an employer place on the taking of holidays?
Restrictions on taking holidays may be expressly stated in the contract of employment, implied from custom and practice or incorporated into individual contracts from a collective agreement between the employer and trade union(s).
Employers may choose to:
shut down for certain periods during which all or some groups of workers have to use their annual holiday entitlement
nominate particular dates as days of closure, when workers are expected to take annual leave (for example, over the Christmas and New Year period)
determine the maximum amounts of leave that can be taken on any one occasion and also the periods when leave may be taken.
Any clash of requested holiday dates may be resolved by management - for example, by considering the particular circumstances of the individuals concerned as well as the needs of the business or by other means such as drawing lots or agreeing 'first come, first served'.
In the absence of an agreement on the taking of leave, the provisions of the Working Time Regulations 1998 apply:
an employer can require a worker to take all or any of the leave to which a worker is entitled at specific times, provided that the worker is given prior notice. The notice period should be at least twice the period of leave to be taken. For example, employers wishing to have a Christmas shut down spanning one week would have to give at least two weeks' notice to their workers."