Compulsory in London from 1875, and other major towns and cities brought in similar regulations.
Slate DPCs are sometimes found in larger properties, in conjunction with cavity walls, but even when installed they can fail due to settlement (Lancashire, Yorkshire, parts of Nottinghamshire, Derbyshire Leicestershire as well as Co. Durham had extensive coal mining which made settlement inevitable). The bit where I am is very hilly, so movementbtends to occur as a result of that. I have seen a number of buildings of this period with traces of a tarred felt DPM or bitumen DPC but after 100+ years these tend to degrade and break down meaning that you effectively have no DPM. You might like to know that until the 1840s there would have been only limited quantities of slate up here because there were few canals, and no railways (and slate is very heavy, and in the Peaks, Pennines and Dales roads were poor quality well into the Victorian era) - and unlike London slate couldn't be shipped in directly by sea from North Wales, etc - so the vast majority of poorer quality houses, mills, etc were "slated" with "Yorkshire greys" aka cleaved sandstone. If you aren't putting slate onto roofs, you won't be using it for DPC
London made its' own regulations, but in general it would take 20 to 30 years for such regs to be taken up elsewhere and in some cases they never were as the London regs were not compatible with local building methods. AFAIK national building standards didn't exist in the Victorian or Edwardian period, and unfortunately most traditional terraced houses are Victorian or Edwardian. You might be surprised to learn that the first national building standards I can find only date back as far as 1965
My house is 1881 and has no slate DPM, the newest house on the street is about 1895 and equally has no slate DPM. Despite this seemingly '"backward" design all the houses on this street from the oldest (1875) to the newest have cavity walls with a stone outer skin and a brick inner skin. When did London mandate cavity walls? 1930s? (Nationally it was during/after WWII).