I'm also originally from rural Scotland (around Tomintoul), for us it was "Breakfast", "Dinner" and "Supper".
Upon moving to the central belt, and ingratiating myself into more middle class circles of friends I then discovered:
Breakfast - eaten usually when you get up in the morning, say early morning until ~1030am
Lunch - eaten from 12pm until ~2:30pm
Afternoon Tea - taken from around 3:30pm until 4:30pm (cakes/biscuits [cookies] and would also include some kind of sandwich - often cut into triangles)
Tea - 5pm until 6pm - usually a lighter two course meal (say an omlette) and some pudding (desert).
Dinner - taken from 7pm until perhaps 9pm - this would be a fully laiden two or three course meal.
Supper - taken from around 10pm until around 11pm (or just before bedtime) - likely cheese on toast or crackers & cheese and a cuppa.
Obviously you don't need to have every one of these meals every day of the week.