In the period of absence from work due to illness no holiday entitlement is accrued, that is correct.
However, there are different ways of calculating leave entitlement, and if you have worked over 1365 hours in the year, you will have the full 4 working weeks allowance. Working 40 hour weeks for 39 weeks is 1560 hours.
So your period of absence may not have any effect in the end; it will depend on how many hours you have worked, when you wish to take your annual leave.