Homemade UK sandwiches are worse than that. In my experience, when most people say "butter" they mean margarine. Shop bought sandwiches are more likely to contain real butter.
You'll either find they don't need a spread at all like https://www.tesco.com/groceries/en-GB/products/253582556 - anything which has mayo on it means they get away without it. The "Just Ham" sandwich from Tesco does though, indeed, have butter.
That's just an arbitrary distinction to avoid calling things "margarine". Margarine had a bad reputation in the UK as something only poor people eat. In other places like the US it doesn't have this stigma and so people feel fine referring to it without euphemisms like "spread".
The EU has very strict definitions of this, and in the UK (for health reasons I expect), there is no market appetite for a spread that contains partially hydrogenated oils.
In the US, regulations are much more lax and substances that are considered unfit for mass market consumption in Europe (typically for health concerns), such as trans fats, are widely available in the US.
You can dress this up as a "stigma" or a "euphemism", but I call it reasonable consumer protection law.
The labeling requirement is theoretically protecting consumers from being ripped off by paying for water when they were expecting fat, but the manufacturers have convinced consumers this is actually a good thing ("reduced fat"). The real reason all spreads in the UK now have <80% fat is most likely cost reduction.
Will everybody on HN please stop assuming the "law" is as it is in their local jurisdiction, and why posting a technical industry article on different fats is meant to convince me I'm wrong in establishing that in the UK (the country in which this thread's food is considered), there are specific terms used for specific products, I can't fathom.
"Products in the form of a solid, malleable emulsion, principally of the water-in-oil type, derived from solid and/or liquid vegetable and/or animal fats suitable for human consumption, with a milk-fat content of not more than 3 % or the fat content.
Margarine
The product obtained from vegetable and/or animal fats with a fat content of not less than 80 % but less than 90 %."
No mention of partial hydrogenation or trans-fats. The only reason UK spreads are not "margarine" is because they do no contain enough fat.
