Restriction on holidays
A. An employer is allowed to specify dates on which you must take holidays and, likewise, dates when you cannot take leave. So there’s nothing generally to stop them restricting your holidays in this way.
That said, there may an argument that your employer’s holiday policy indirectly discriminates against women, who still provide the majority of childcare (including during school holiday months). In that case, they would need to be able to objectively justify this rule.
Your employer must also aim to make sure you take your statutory annual leave during the current leave year.
If the restriction means you will not be able to take all your leave during the current leave year, you may be able to bring a claim under the 1998 Working Time Regulations.