What you call a meal depends on the TIME you eat it, and to a lesser extent, what you eat. It has nothing to do with the place.
Q: As I know you have breakfast at home before going your job.
A: In fact, you can have breakfast anywhere. It's just the first meal of the day. It has nothing to do with going to work.
Q: But do you say "I have breakfast" when eating after getting your job place (say, office) before starting your work time or at home on weekend?
A: Yes, definitely. We have breakfast every day, wherever you are. Usually at home, but it can be at a hotel, at work or in a cafe.
Q: When do you say "I have brunch"?
A: The word 'brunch' is a combination of 'breakfast' and 'lunch'. It's a larger meal than breakfast, but usually a lighter or more relaxed meal than lunch. It's something that you'd have late morning - maybe around 10 - 12.00. You might have brunch if you've had a very early breakfast, or no breakfast at all, and you want a meal mid-morning, but it's too early for lunch.
A: And as I know the Englishmen says "5 o'clock" term.
Do Americans say that?
No. Nobody says that. In fact, hardly anybody says 'Englishmen'. Your English book must be at least a hundred years old! Whoever told you that this is a term to refer to a meal was wrong. 5 o'clock is a time, not a meal. Nobody, even in England, would understand you if tried to refer to a meal as '5 o'clock'. What you are thinking of is 'tea'. In the past, upper class people would have a light meal of sandwiches and cakes with tea at around this time. You can still get this in teashops and hotels in the UK, but it's generally served a little earlier in the afternoon. And do try to avoid the term 'Englishmen'. This refers only to adults of the male sex from the relatively small country called England. If you are only talking about England, excluding Scotland and Wales, you could say 'English people' or 'the English'. Otherwise, you should say 'British people' or 'the British'.