As is shown by the answers ths far, people tend to be split into two camps. Dinner= lunch, tea =evening meal Or
Lunch= lunch, dinner= evening meal.
From my personal experience, the half of my family from the midlands and Northern England use the former, and the half from the South of England use the latter.
I'm firmly from the SE, and i've always used lunch at midday and dinner in the evening.
I suspect the differences probably stem from regions and the eating habits brought about from the different industry prevalent in different regions. i.e (historically) more heavy manual industry in the north, which required larger meals in the daytime to keep the workers going, thus the main meal was at midday and was named dinner. The lighter meal was eaten when the workers got home, after the days work was done, and was tea.
In areas of lighter industry, non manual labour, a lighter meal was sufficient to keep one going during the day, and was lunch, the main meal was then eaten after the days work, and was dinner...