Dinner is the main meal of the day, whether it's eaten in the middle of the day or the evening
Hence school dinners (hot food) served by dinner ladies - as the hot meal is the main meal of the day
A Sunday roast is the main meal of the day, as is a Xmas roast - so they're Sunday and Xmas dinner.
But sandwiches are a packed lunch - because it's assumed there will be a hot main meal in the evening
Most people wouldn't call a sandwich and packet of crisps a packed dinner
The main meal of the day is generally in the evening, so it's breakfast / lunch / dinner.
Tea is a light meal around 5-6 that a small child eats, if they eat separately from the adults.
Dinner is the main evening meal. 6pm onwards.
Restaurants have lunch and dinner menus
They don't have dinner and tea menus
ACAS and the UK government refer to the midday break for workers as a lunch break