When you say "a PhD in Law", what area specifically are you looking to get it in? There probably isn't really a general equivalent because it doesn't really...make sense for there to be one. A university that might be excellent in one area of legal research and academia may be highly deficient in another. Given the extremely specialised nature of the PhD, generic metrics aren't relevant - those don't tell you if the world leader in research in a given field is working there.
Generally, all universities in the UK are research universities, and law "schools" lecturers in the UK tend to research first, teach second. So, any "highly ranked" law course provider will likely have some research ongoing that is of fairly notable standard and might be worth considering (e.g. Oxbridge, LSE, UCL, Durham). For legal philosophy/jurisprudence Oxford is, as I understand, the world leader - they're also considered one of the, if not the, strongest law departments in the country as far as undergrad studies go (and have a very prestigious masters course, the BCL). As I understand Cambridge has a major international law research centre, and a lot of land/property law stuff (in the department of land economy).
Durham or UCL are probably the "next step down" for undergraduate law I guess from Oxbridge/LSE, but that's as above not really relevant to the quality of a PhD programme in a particular area of research. If you wanted to research e.g. Islamic law then SOAS would probably be a strong option for example, despite being considered rather average by undergrad generic metrics.