I was led to believe that the milk first or milk second debate was more an issue of upbringing and organic to social class. Standard middle class habits tended to dictate that the milk be poured first. This could have been due to the use of china of moderate quality and its reduced resistance to staining when the tea was poured in first. It also could have been due to an attempt to distance themselves from the lower class habits of using non-porcelain based cups, tins, etc. which were not susceptible to cracks or stains and so the tea was poured first.
Established middle class and upper class traditions would call for a milk second approach. Ironically, the same order as the lower classes, although the reasons were different. The quality of china present in these wealthier households would have been more resistant to staining and therefore not adverse to tea first.
As far as the theory that 'milk first' originated when cups were made of soft porcelain; I believe that has been discounted. Any household that was using porcelain cups would have been serving tea from a teapot. After allowing for normal steeping times, although still quite hot, the water would no longer be at a temperature that would threaten even soft porcelain. The theory was first postulated by Samuel Twining and was therefore held as gospel, but is more likely than not simply folkloric myth. Both hard and soft porcelain are resistant to the temperature of steeped teas.
From the twentieth century onward, the serving traditions became regionalized. Areas with a larger working class population likely held preferences for milk first. Whereas, more affluent areas espoused a milk second tradition. After all, it was not too long ago where the expression "she/he is rather milk-in-first" was used to refer to people of a lower social standing than oneself.