London

London tap water is hard. Very hard. High in calcium, high in bicarbonate, and that bicarbonate is doing most of the work when it comes to how your coffee tastes. It buffers acidity, which means the bright, fruity notes in a light roast get flattened. What you're left with is body, weight, and a tendency towards bitterness.

This is why London's coffee culture developed the way it did. For generations, the default was dark-roasted, strong coffee, served with milk. The water suited it. Dark roasts have less acidity to begin with, so the high alkalinity doesn't strip out character the way it does with lighter beans. Add milk, and the heaviness of the water becomes a virtue: it produces a dense, full extraction that carries through dairy.

If you're brewing espresso-based drinks with dark roasts and milk, London water works. For filter coffee or light roasts, it's working against you. The mineral load is too high, and the balance is wrong. That's not opinion: it's what the chemistry does to extraction.