Don't they almost all work on a "goodybag" model now where you top-up £x for the next month of tens of GB of data, hundreds or unlimited minutes, and unlimited texts? Using "real" balance is uncommon in my experience

Only if one buys such a "goodybag".

My father doesn't and so pays PAYG rates when he uses his. Which probably makes sense for him, given his infrequent use pattern.