Yep, in England you have to have the cup for ordinary (black) tea, but strangely enough it doesn't apply to herbal teas, so you could ask if someone would like a mint tea for example, otherwise it is a 'cuppa'...mine's a builder's (very strong)!....[sorry a little flippant : ) ]
You guys are just going to have to accept that the grammar in Portuguese it's different from the grammar in English.it may not seem correct or grammatically correct when you look at it from the aspects of English grammar but it is completely correct when you look at it from the viewpoint of Portuguese Grammer. There are going to be many differences that do not make sense if you compare it to English grammar but you're just going to have to accept that a different language with different grammatical rules.
If it says um cha, I am going to write a tea, and then I am told I am wrong...?!!