I agree you might have to narrow it down and talk about limitations. There will be PhDs registered as MPhil for their first year-do they count if not upgraded? There will be people who have stopped getting funding but who are writing up, or people working but waiting for viva. What about people that drop out early? That confuses which data you include e.g. full-time, part-time, 1st years, 5th year on extensions? Industrial PhDs, researchers working in the same lab and studying part-time?
The universities may not be able to even keep tabs on this information! I've been on a faculty committee looking at departmental statistics..!
Do you actually just want a cross-section or to design a questionnaire to collect the data? You might need to define what you want more clearly in order to get the right answers.