So does it work like this now? (I’ve lived in northern and southern England, and for a long time now in Scotland).
Northern and/or more working class - Dinner (middle of the day), tea (evening meal), supper (not a term I heard a lot but my gran used it and it meant a hot chocolate, or a slice of toast if you were still hungry). This one is how I grew up.**
Southern and/or more middle class - Lunch, dinner (evening meal), supper (still a late night snack but maybe a slice of toast from a crusty loaf!), tea (literally a cup of tea, maybe with a slice of cake). This one was the most common among people I knew at uni - evening meal was definitely dinner.
Very posh - Lunch still middle of the day. Tea could be afternoon tea - full on or a simple cup of tea and nice cake. Then - and this is where is gets complicated and I could be wrong! - dinner is the evening meal, but is called that when it’s more formal (think Downton Abbey) and/or when there are guests. Supper is the full evening meal when it’s more informal and casual; a ‘kitchen supper’ as mentioned above, means not eating in the dining room/getting out the silverware.
Scotland - Some of the above apply, but ‘supper’ is a large takeaway, usually fish and chips (a ‘fish supper) eaten on a Friday/Saturday/Sunday.
**Historically, my understanding is that among the working classes tea and supper were both smaller meals. Your main meal would be dinner (at lunch time - maybe provided by your factory owner if you were lucky), tea was literally the drink plus a snack if you were lucky. Supper was much later and similarly small - maybe a slice of bread and some Bovril. Plus, if you were struggling, you might have supper but not tea. Would love to hear from a food historian if that’s right.