Hmmm. There are some strange answers above!
If you're off sick over a holiday period that time counts as 'sick leave' and NOT as 'holiday' (unless you choose otherwise); so the holiday can still be taken later on. Even if you're actually on holiday and you then fall sick you can still ask for the days that you were ill to be counted as 'sick leave', giving you additional holiday time later on.
Your contract of employment CAN'T change those rules, which are defined by law. See 'Sick leave and holiday' here:
https://www.gov.uk/taking-sick-leave