Fascinating thread. Full of the usual snobbery, ignorance, prejudices and humour that I come to Mumsnet for!
My take from this is that some southern English people say 'cuppa' and some Northern English people say 'brew'. We Scots mostly don't say either, but will say 'a cup o' tea' which might sound like cuppa but isn't the intention. The 'tea' part is almost always there.
My granny used to say 'tassie o' tea' (old Scots word for cup) and I sometimes say this to wind up the DC.
Re. the meal, I've read that the definition of dinner is the main meal of the day, and for workers that was usually midday, to give them sustenance throughout the afternoon's labour. They then had something lighter when work was over, around the time when posh people were having their afternoon tea. This tea-time meal became shortened to just 'tea'. The posh folk had their dinner later in the evening. Supper is defined as the last meal of the day, so that might be your midnight snack, or your dinner, or your tea, if you don't have anything later.
I use all the terms - tea if I'm eating early, supper if I'm eating late, dinner if I'm eating out. A supper (with article) or suppers (plural) if it's from the chippy. (White pudding supper - delicious!)
There's no right or wrong, despite what others declare. To each their own!