Brit here, an average cup of coffee would be around 1 heaped tsp of instant coffee in a 300ml mug with a bit of breathing room, say 275ml? But that would include the milk, so maybe around 200ml water and 75ml of milk. Would vary quite a lot from person to person though.
That'd be around 57 mg caffeine. On the weak side, compared to espresso or drip coffee. You think the most common preparation is instant coffee? Kind of surprising.
In the UK? Yes. I think in the UK, most people drink most of their coffee at home rather than out at a Starbucks or similar.
Some folks have a percolator or a cafetière or a fully automatic, especially younger people, but if I were to be invited round to meet someone’s parents, I’d expect instant.
To be fair, I haven’t lived in the UK for over ten years now, I can imagine Nespresso type machines may have taken the lead.
