In my view, this is an unfortunate necessity unless you're one of those people who enjoy breakfast cereal without any sort of milk or hot drinks without milk. For me, when I camp in or near a shop I'll always buy a pint of real milk. In winter or cooler weather, it'll keep for a day or so and I'll carry some in a plastic bottle. For those other times, such as the Challenge, powered milk is the thing. Forget Marvel though - it tastes disgusting and smells awful. PlusPints is OK but it's dried skimmed. For me, a product I only discovered in the last year is Nido, made by Nestle, but not always easy to find. Tesco stocked in their larger stores at £3.79 for a 400g can but now seems to be discontinued. Asda advertise it at £3.00. I've just taken delivery of a six pack of it from Amazon for £18.00 with free delivery so I'm very happy.
Nido is full cream powdered milk. It tastes creamy, is good on muesli and dissolves well in hot or cold water and hot drinks.