I have to be honest and say I don't know that it's always cheaper when you're filling hungry teens to cook completely from scratch.
For example. A jar of value curry sauce is 20p. Add some veggies and some rice and that's dinner for 2 teens. I don't know that I could make the curry sauce for 20p by the time I'd have spices, coconut milk, yoghurt, whatever. And also, for teens, it needs to be insta-food. It has to be ready in under 10 mins or else there is NOTHING TO EAT IN THE HOUSE EVER AND THEY'RE STARVING.
A lot depends on what your child's allergy is though.
For example, could you buy cheap value pizza and freeze and get them to top it more with whatever they want? That's what I do - as cheap as buying bases and all they have to do is add whatever they want and some extra cheese.
I do buy el rubbero cheese for melting/cooking and keep the good stuff for when it will be noticed too.
Value tinned toms, kidney beans, packets of lentils and soup mix. Big bags of rice. Bags of spuds from the wee man at the side of the road - but they aren't always great because sometimes now we don't eat them up quickly enough.
Also I rarely use kitchen towels. I use old nappies as cloths and do a hot wash of them once a week. I do use Ariel washing powder as nothing else gets grubby sports kit clean, and I splurge on a branded fabric conditioner when it's on offer.
WRT to bubble bath/shower gels - fancy ones for presents, the rest of the year it's cheapies from Lidl or the value ones from Tesco. You don't like it, go buy your own.
I don't know that it's cheaper to make bread for example. Value garlic bread is 30p or thereabouts. Cheap loaf Lidl fake of best of both is 59p. I don't think I could make a loaf for that. And with that I get sarnies for lunches and cheese toasties.