Just catching up on the many responses and feeling so grateful for the good advice and insights.
I'm not sure my updates and responses to questions are visible on the thread (I can see them but, reading some posts, I wonder whether other posters are able to?), but I'll have a go at responding to a few more questions and comments in the hope they add to the picture.
They're young teens. Not 21 -not sure where that came from.
Neither child is overweight.
A snack shelf or cupboard is a good idea. I always resist 'when it's gone, it's gone' as it might not work out fairly, but it is probably sensible.
It's not that I object to them eating protein, it's the fact that they help themselves to main ingredients and don't tell me.
DC1 is autistic and has some sensory seeking behaviours around food, hence the popularity of spicy condiments.
Both DC come food shopping, not always, but they know the drill. I usually let them take a basket and give them a £10 budget -they can buy stuff which is 'just for them'; crackers, frozen spring rolls, tins of soup (DD1!) etc.
Smoothies and energy balls: yes, they love making these and experimenting with the 'add whatever you have to hand and see what happens'.
They genuinely love frozen peas, sweetcorn and berries and I'm not annoyed about them eating them, but rather that they finish the bag without telling me, and also leave the bag empty in the freezer. It's mainly DC1, who finds properly disposing of rubbish an irritating and irrelevant task, interrupting her 'flow'.
Baking and cooking: both DDs are good cooks and they got that way because I do let them cook and experiment with recipes and ingredients. They enjoy preparing food, so I think some of their cooking and baking is for entertainment rather than feeling hungry. DD1 seems to bake all the time when she has friends over, so @mathanxiety is right to observe that there is an important social aspect to being able to host friends in way that includes sharing food in a fun and generous way.
I think a lot of my OP was about feeling aggrieved that, despite trying to accommodate sensible snacking and build autonomy around food with tuck allowances and their personal supermarket budget, they still give me both barrels about the abundance of UPF snacks at their friends' houses. And see the all the contents of the kitchen as fair game.
I don't have a choice about the food budget -it is what it is. I do have to take care, it's as simple as that. And no, there isn't anything else I can feasibly cut back on.