My UK experience is that for many people it is dinner, not lunch, that is the main meal of the day. You might have a sandwich or something similar that you can eat in the break room at work, and then go home for a cooked meal with the rest of your family in the evening. This isn't universal, of course -- I have my main meal at lunch because I can get a cooked meal in the work cafeteria and then I don't need to cook for myself at home. But in the past when I've worked places that don't have cafeterias I've eaten sandwiches and had the main meal in the evening.