Thanks for the suggestions. The instant coffee itself isn't a problem for the timescales I plan for, but I've never found a powdered or tinned milk that tastes close enough to fresh milk. Since it's primarily a comfort food, taste is important.

My current approach is instant cappuccino sachets. It's not quite as good as fresh, but it's good enough that I put off buying milk knowing I've got something to drink. It's relatively expensive, though, and its shelf life is 6-12 months which is too short for me to consume a large stockpile normally. I keep a few week's supply anyway.