Unless your contract specifies otherwise, the only legal obligation your employer has to comply with is to give you the statutory minimum number of days holiday. The employer is entirely free to choose those dates (as long as you're provided with adequate notice of them). You have no right whatsoever to any choice about the days when you take your holidays.
For example, an employer can say that his employees will be given the first three Wednesdays in January as holidays, followed by the second and third Tuesdays in February, the last three Thursdays in March, etc, while insisting that employees work (for normal pay) on Christmas Day, Boxing Day and on all other public holidays. As long as the total number of days of paid holidays in the year comes to 28 (for someone who works 5 or 6 days per week) statute law will have been complied with (even though no employee would ever have more than 2 days off work at a time - there's no statutory right to whole weeks of holiday). Only individual contracts can give employees additional rights.
Chris