Regional differences. apply with this. I'm in the South East. Now lunch is middle of the day, can be light, soup or a sandwich or a full meal, as in having 'lunch out' restaurant meal. Or it could be brunch, late breakfast early lunch which would suffice till dinnet. Tea would be the middle of the afternoon, cup of tea, slice of cake, more occasional. Or children's meal after school that would be tea if they eat later, after 6 then it's dinner. We usually eat the main meal, dinner in the evening around 7ish. Roasts on Sunday with the family are called lunch even though we eat those around 4ish.
Back in my childhood it went like this. On school days the dreaded school dinners were served at lunchtime. Tea a meal between 4 and 5 at home. Sunday roasts middle of the day those were called dinner, tea lighter meal around 5ish, but if we had an evening meal at home later it would be dinner. Something cooked at tea time on Saturdays, not as heavy as a dinner, but more than tea, treat type of food which for me were chipolatas and grilled tomatoes and something nice afterwards whilst watching "Thank Your Lucky Stars" that would be supper.