The first point to consider is when your 'working year' actually is. Many employers calculate holiday entitlement based upon a year starting on 1st January and ending on 31st December, but others work on the financial year or use the anniversary of the employee's commencement date as the start of the working year.
However, assuming that your holiday entitlement is calculated on a calendar year, you're entitled to a minimum of 28 days paid holiday (including all 'enforced holidays' such as Christmas Day or other days when the business is closed) during that year. That assumes that you work a 5 day week. Otherwise your entitlement is calculated as 5.6 times the number of days you work each week, but capped at 28 days.
Your employer is obliged by law to ensure that you get the minimum number of days holiday within the current working year. If circumstances prevent you from taking those holidays, you must be permitted to carry those holidays over to the next working year. However such circumstances shouldn't arise through the employer's actions (or lack of them). They'd normally only apply if, for example, you couldn't take your holidays because you were sick.
If, by the end of your working year, you'll have already taken your statutory number of days of paid holiday, then it's contract law (rather than statute law) which applies. You need to read your contract (or 'statement of employment particulars') carefully, to see exactly what the wording says about holidays.
Chris