Are biscuits ultra-processed?

Not always, and shortbread is the clearest counterexample: flour, butter and sugar is a group 3 product with three ingredients, all of which a kitchen has.

What moves most packets

The fat is usually the answer. Most packet biscuits use palm oil or a blend with emulsifiers rather than butter, because it is cheaper and more stable. Add raising agents beyond plain bicarbonate, flavourings, and often invert sugar syrup or glucose-fructose syrup, and it is group 4.

• Flour, butter, sugar, and possibly salt: group 3.
• Emulsifier, flavouring, invert sugar syrup, colours: group 4.
• Anything coated, filled or sandwiched: group 4, effectively without exception.
• Digestives are usually group 4 despite the short-sounding name, from the fat blend and raising agents.

Biscuits are a good test of whether somebody has understood the classification, because the intuitive answer is that all of them are and the accurate answer is that the plainest ones are not. Same shape as crisps, where plain salted is group 3 and every flavour is not.

Working this out from a receipt is harder than from a packet, because a till prints about twenty characters of product name and no ingredients list. That is the problem Bagged exists to solve, and how a receipt is structured explains why it is the hard part.