If I'm out and about and a warm beverage or two is on the horizon (probably best to say when not if at this point!) then I often freeze a third of a bottle of milk the night before and top it up with fresh milk in the morning.
It is really effective in keeping the milk cool as you've essentially made a large milky ice cube and it means that you have usable milk whenever you need it, as opposed to having to wait forever for a fully frozen bottle to defrost). You need to remember to freeze it with the milk away from the neck or you will create a bung which prevents you filling it. It's also useful for something like water in a water bottle too.
These coffee sachets (which are available from Cotswold Outdoors and Go Outdoors) are great but need fresh milk in my humble opinion...if one is doing posh filtered one doesn't need powdered milk!