Holiday calculation problem for a departing employee

I wonder if I could have some guidance in calculating the remaining holidays for an employee who has chosen to terminate her contract and will be leaving the company at the end of September.

The employee is entitled to 31 days holiday this year (20 days + 11 public holidays).

However, this is made slightly more complicated by the company requirement (Employee Handbook) which states that employees are required to take 1st January, 25th December, 26th December as compulsory holidays as the office is closed on those days. All other public holidays are flexible – employees can either take them when they fall or take a day in lieu if preferred, with their manager’s agreement.

So, to get to the point, as this employee will be leaving at the end of September would her holiday entitlement for the nine months she has worked be 9/12 x 31 days or 9/12 x 29 days?

In other words, is it correct to exclude 25th December and 26th December from the entitlement?