best of luck, i'd suggest hanging out with them and directly asking them questions like, how's your phd going, is your experience in the UK/uni/department what you expected? leading questions like those. hope that helps.
am an international student, but i've done all my uni studies here, so i might not be able to relate to what new international phd students experience coz i haven done uni anywhere else.
i would expect answers like communication issues, cultural differences, dilemmas in how to relate to supervisor..stuff like that.
when is your interview? i could ask around..