Ahh OP, I hear you. It's a minefield at times. Here's a list my German pal and I came up with when having this exact same chat, copied and pasted. (disclaimer - I am Scottish, living in Scotland and grew up working class but with an abiding love of Jeeves and Wooster. I am probably now middle class)
Breakfast: First meal of day. Can range from cereal / bread products to full cooked breakfast with various meats and eggs. Generally does not involve cheese
Elevenses: Eaten at around 11am. Usually with cup of tea, would consist of light snack or perhaps a cake.
Brunch: Bastardisation of a meal. Can actually be eaten later than traditional lunch (e.g. up to 2pm), if it's your first meal of the day. E.g. a hungover brunch. May involve American-type foods such as waffles with syrup, or can be savoury. Smashed avocado on rye bread is a stereotypical hispter brunch.
Lunch OR Dinner: Midday meal. Lunch always means midday meal. Dinner is more fluid. Schools will often refer to "school dinners". If being referred to as dinner, it is generally hot and the largest meal of the day. However lunch can range from sandwiches to a full meal too. A "roast dinner" can be served anything from midday to evening depending on preference, and is often a celebratory type meal or eaten on a Sunday.
Afternoon tea: Eaten midafternoon. Consists exclusively of small finger sandwiches, cakes and scones. "modern" twists include tiny sausage rolls etc. Cups of tea accompany this. Not a standard meal. You would either invite people over for this or go out for it.
High tea: subtly different from afternoon tea, though also served in the afternoon. Includes hot food as well as afternoon tea fare (this may be a Scottish differentiation). Sometimes has childish connotations, as may be eaten approx 4pm - as children's main afternoon meal, they will be in bed later.
Tea: Evening meal (if you aren't in the South). Standard hot food. Can be the main meal of the day. If a child asks you if they can go to a friend's for tea, they mean t he meal. If a friend asks you over for tea, check what time - if they say evening, they plan to feed you.
Is also known as Dinner.
Dinner: eaten later, usually after 7pm. You might go out for dinner, or hold a dinner party. Usually has multiple courses.
Supper: a late evening meal, before bed. Usually more casual than dinner. Can vary from light snack food to a proper hot meal. Posh people might invite you for a "kitchen supper" - this means they plan to feed you in a relaxed manner in the kitchen, not in the dining room. They might invite you round for 8pm and feed you closer to 9.30.
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You asked about "munchy". The munchies are feelings of hunger, outside of a standard meal time. You might not have eaten enough, you might have smoked too much cannabis. I
f you ever find yourself somewhere selling a "munchy box", under no circumstances purchase it. Your arteries will thank you.