Have you tried looking for growers feed rather than all flock.Watermelon is stays okay if kept cool, a bit like apples.
I think its because the chickens are not so thirsty anymore. In summer they do eat it gladly.
Here its only possible to buy gmo free if you buy organic. And there is no organic all flock.
I think the price at the mill where I buy is fine. The agri/pet/gardening shop is much more expensive, especially the organic starter. They ask € 13,50 for 5 kg.And don’t sell larger bags.
For example.
https://www.smallholderfeed.co.uk/products/poultry/poultry-grower-pellets/