Hello, welcome to the website. My name is Tony, I can assist you with this.
The Working Time Regulations do permit an employer to require you to take holiday during a notice period, and indeed, the employer can say when the holiday should be taken.
However, it depends what your contract of employment says about this too, although if like most, I doubt it mentions it at all. If that is the case, then the employer can require you to work out your notice period and to take part of that as holiday that you have accrued.