I've been playing around with cooper's kits since buying their "micro brew kit". It'll be replaced in this country in May with the "DIY" kit. I like the "micro brew" FV better,myself.
Anyway,The cooper's beer kits are indeed pre-hopped. So do not boil them,or you'll loose some of the hop/flavor profile. I make a "hop tea" in 1 1/4G-2G of boiling water for 15mins. Take the BK off the stove,pull the bagged hops out & let drain with sanitized tongs. Then mix in some DME (to style,if so desired),then the cooper's can,brewing sugar,whatever. Stir very thoroughly to incorporate. It'll foam up a little,but that's normal. Slap a lid on it & allow to steep for 15mins or so while sanitizing the FV,etc. Then mix to 23L (typical cooper's wort amount).
Some have even cut the total amount down to 20-21L to get more body/alcohol. But,since I like to add other things (like DME,hops,maybe a little brewing sugar),I go all the way to 23L. I still get good beer.
By the way,the cooper's brewing sugar is better than the 100% dextrose,imo. It's 80% dextrose,20% maltodextrine. The maltodextrine gives a bit more color & head retention,& mouth feel. I got the brewing sugar included in the beer kit that came with the micro brew kit. Def better,as my avatar shows using the cooper's OS lager kit/brewing sugar.