These confusions aside, names do vary across the country, and other countries (eg Scotland) do different things. In London, dinner is an evening meal, later than tea.
The lines are just abandoned in London. A caff (cafe) may open from 7am to 2pm, a restaurant from 11am to 3pm then 6pm to 10pm. Another may be open all day, serving an all day breakfast. There are places that do 24 hour service. And Gregg’s and McDonald’s.
The point is, it’s fine to eat whatever you want whenever you want. If you’ve just arrived in London from a great distance and are jet lagged, and fancy a cooked breakfast at 4pm, you can find it and enjoy it. Though we always find the best way to overcome the lag is to fit into local patterns of food and sleep as soon as possible.