Oh, MissKoo, you have asked a highly charged, technical question! There are class differences at work, and regional ones and also age ones
I'm 54, from the SW of England and have tea about 6. It's the main meal of my day. When I was growing up, my dinner (midday meal) was our family's main meal.
My partner, 43, from the NE of England (steel town) calls this 6 pm meal 'dinner'.
My upper middle class friends, 53, from the SE of England call this meal 'supper'.
You've had info from Scotland above.
Afternoon tea is the scones, sandwiches, cake and tea (drink) event which happens rarely now, in my experience, but is fun when it does.
I have never heard anyone use the term 'high tea' except N Americans. But I did read it in books set in Northern England when I was a child. I got the impression it involved drop scones (griddle cakes), jam, ham, fruit cake and a lot of other yummy things. We used to have this kind of 'tea' when I was a child but it was never 'high'. Someone may be able to shed more light.
For tea tonight (just finished), my mother, son and partner all had fish and chips from the chippy. Most unusual for us but it's been an odd day. I had raw baby spinach, tomatoes, a tin of mackerel and a few cubes of feta. But we're at Mum's where we still have the main meal in the middle of the day. .
Great question!