Hi,there is only one wormer licensed for chickens Flubenvet.You will need about 30kg of feed,by far the cheapest way to buy this is pre mixed feed from Marriages,it works out about £11 a bag you will need 2.
A 60g pot enough to treat on 20kg bag of feed is £13.20,so the pre mixed is far cheaper,a larger 240g pot is £44 this will treat 8 25kg sacks of feed so still works out at £5.50 + cost of bag of feed.
Some people use other unlicensed products.
Graham.