No: there is no obligation to do so. Some employers develop their own policies, permitting some flexibility (for example, holiday for one year cannot be carried forward beyond the end of the following March).
Employers may not demand that employees carry forward any part of their statutory holiday, and must ensure that at least four weeks' paid holiday are taken in each leave year. However, both sides may agree that the extra eight days' of paid holiday now being phased in (see question 1) shall be carried forward or (until 1 April 2009) may be 'bought out'.
