I picked up a good tip on ukcs. If you are taking a cool box (with freezer blocks or ice from the supermarket) take some bubble wrap with you. Push it over/around the food you want to keep cool so you are not chilling the top wasted space of the cool box.
All the sites I've been on have put the blocks in the freezer for a small fee so you need two sets so you can swap them over every day. That should be enough to keep milk etc fresh. Even better with the bubble-wrap, I'm definitely going to pinch that idea!
You may want to make your blocks easy to identify if you want to be sure of getting the same ones back (ours are nice big flat ones so I don't want to be lumbered with the squarer chunky ones). Some people put them in coloured carrier bags, we wrap multi-coloured elastic bands round them.
Lol, the site we usually go to has a stream running through it. If you look along the bank on a warm summer evening you will see a row of wine bottles on lengths of string dangling from the adjacent tents/caravans into the cold water .
Our cool box didn't quite last the weekend but it was very very hot and DH annoyingly left it in full sun for half of one day. Even so the milk at the bottom was ok, it was just the butter on the top that melted over everything. I have heard good things about the Coleman Extreme cool box though so I would like one of those as we are not planning on EHU.....for now.....
We don't use either. Milk's the only thing we find which tends to go off quickly, and if you buy milk from the campsite or local shop in smallish quantities, you don't have a problem. also you can use UHT milk if you don't mind the taste.
One thing we do is take frozen food which will take a day to defrost, typically, and put that in with the dairy and other food which benefits from keeping cool.
we don't take meat so that's not an issue, but even if you normally eat meat, if you take veggie sausages etc there is less problem about keeping them cool.
We use a cool bag when we aren't doing ehu. and just change the blocks regularly. We take the second nights dinner frozen as well which acts as a cool block. If we can't change blocks then we buy something frozen from a supermarket when out shopping