Spending £3.5k on topping up your child’s lifestyle at uni is not remotely in the realm of necessary or typical spending, even with twins.
In most cases, if the student is on the minimum maintenance loan, that's possibly the least you can expect to pay. Assuming that you can find accommodation around the £5k/year mark (might be possible in Sheffield but unlikely in Bristol) that £3.5k leaves around £100/week for food, going out, toiletries, travel home, clothes, stationery, clubs/gym etc. Yes, it's doable but many of us are subsiding university by far more than that just to pay for accommodation never mind anything else.
And as others have said, p/t, gap year and holiday jobs are harder to find.
In my experience, the nursery years were horribly expensive (although it might be a bit better now with the free hours). It then felt manageable for a bit before it slowly ramped up as food, holidays, clothes, tech, clubs, meals and days out, trips, uniform, sports kit, driving lessons all added up. And then you meet the university years!