Legally, I do not know, but in my company you simply get paid as normal for your sick days, and your Holidays are given back to be taken at a later date.
In the royal mail, if you are off sick with a doctors note, you can get the holday rescheduled. However, you said short term, so I assume you mean just a few days with no note, in which case I should imagine you'll just have to swallow it.
sorry.... just re read your question. You are off sick when your previously booked holiday days start. Assuming you have a sick note from your GP you should be entlted to the holiday days back. If you are not in a sick pay scheme however you won't get paid.