Tea is either a drink or a small meal at 4pmish.
We always used to have 'tea' at my nans, sandwiches, cakes and a pot of tea.
If the children's friends come over to eat, they have 'tea' whereas DP and I will have 'dinner' later.
Normally we all have 'dinner' together at about 6pm, except weekends, when we sometimes have 'tea'(snacky, buffet type thing, agree with uqd, a proper meal like lasagne or sheperds pie, is not tea) and 'supper'(small meal after 7pm).
A meal in the middle of the day is lunch, unless it's a full English at 11am, like on sundays, then it's brunch.