Unfortunately unless your employer has agreed to allow you to swap then you are not entitled by law to take your holiday at a later date and must take the allotted days as holiday even though you were sick during this period.
However, you cannot claim both sick pay and holiday pay for the same period. So if the period of absence has been treated as sickness absence already by the payroll department, then you would have a stronger case for taking your holiday at a later date.
See also: Holiday leave and pay – the law