Your question prompted me to look into it, and it turns out that I was partially mistaken when I wrote this:
From Victorian times to the present day, this is true only for the British working class, particularly in the North. The upper class, regardless of what region they are from, would never call their mid-day meal "dinner." Sorry for this mistake. I am but a lowly peasant who knows nothing of the ways of the wealthy.
To answer your question, modern working class Brits in the northern regions really do call their mid-day meal "dinner." Their early evening meal is called "tea." It's just a light meal, such as a sandwich. Before bed, they may have another snack called "supper."
In modern times, Southern working-class Brits and posh Brits from anywhere call their mid-day meal lunch (or luncheon if they want to be fancy), which is light. Their evening meal is called "dinner," which is a heavy meal. For posh people, this may consist of several courses. If that isn't enough, they may have "supper" before bedtime.
Generally, "dinner" is what the heaviest meal of the day is called, regardless of where you're from. In most of the U.S., "lunch" is the mid-day meal, and "dinner" is the evening meal. In certain regions of the Southern states, the mid-day meal is "dinner," and the evening meal is "supper."
Anyhow, I've deleted the above highlighted sentence from my scanlation notes, and I corrected the first panel of page 15 to refer to the meal the students were having as "luncheon."