Lets look at their "Classic Box" (https://www.hellofresh.co.uk/food-boxes/classic-box/), which is £39 a week for three meals. I priced up what I could see of the box contents using Hubbub (https://beta.hubbub.co.uk) - largely because I work there, and so know my way around, and it came to a little over £20, and that's without any real effort to match the portion sizes they're shipping, I could probably get it down to more like £15 just by buying just the amount of each ingredient needed for the meals being made, and going direct to the producer rather than via high street shops who are going to be adding their own margin.
That leaves, pessimistically about £20 to cover delivery, and the customer's share of central costs such as recipe development, packing, and the usual business expenses of a web based company. At the sort of volume they're doing, I could see them easily making 40% profit, and potentially more than that.