The market research company YouGov has published a report on that most fascinating of sociological subjects, what you call your main evening meal.
The basic answer is that north of the Trent it's tea and south of it dinner, but there are lots of class and other differences beyond that.
As you'd expect, I'm a breakfast dinner, tea man, but Northern people who are trying to sound posher than they actually are occasionally called their midday meal lunch, and in the South there's a tendency for middle-class people to call their evening meal supper, an aping (unconscious or not) of the aristocracy who apparently call a formal evening affair with guests served by the staff dinner, but a simpler meal without guests prepared by the butler for them in their kitchen, rather than the dining hall, supper. Supper to me is a light snack - crisps, cheese, toast - just before bed so I'm not sure what people who call their main evening meal that call it, a late supper maybe.
And then there's brunch, a cooked or cold meal eaten later than breakfast (the only meal name incidentally which everyone can agree on), either in the late morning or early afternoon, although I'd call the former a late breakfast and the latter an early dinner (a name which also extends to so much else: school dinners, dinner money, dinner hour, all referring to the midday meal, whether a hot main meal or a substantial snack such as sandwiches or something cooked on toast).
Afternoon tea falls between meals, ranging from that beverage with biscuits or cake to a more formal event with sandwiches, scones (however you pronounce that!) and even smaller cooked items like crumpets or toasted teacakes (another linguistic minefield), eaten between dinner and tea at around three or four o'clock.
Much of the confusion around this question stems from the fact that originally everyone, of whatever class and whether they lived in the countryside or the city, ate their main meal in the middle of the day and called it dinner, a tradition which persists in schools and was once prevalent in factories with subsidised canteens, especially in wartime (I'm not sure why in the South the name of the main meal has followed it to a much later hour, and in the North become tea, which would once have been a simpler repast between five and seven o'clock). As Christopher Hibbert says about a country house in the early eighteenth century in his 1987 book The English: A Social History 1066-1945:
"Breakfast was served at about half past nine or ten, and usually consisted of tea or chocolate and hot buttered bread, perhaps with cheese, or toast...
Dinner was served at about four or five and supper at ten, though by the early nineteenth century the hour of dinner had moved on to nearer seven o'clock, and luncheon was served as an additional meal in the middle of the day....In simpler homes dinner was still served in the middle of the day, although the provincial family with pretensions might sit down at two or three o'clock".