My understanding is that the “lawyer” in question wishes to sue (for defamation) someone who posted on mumsnet (or “nuttersnet” as they so charmingly refer to this site). In order to do so, they need to know who that person is.
Quite rightly, MNHQ won’t release any details unless ordered to do so by a court. The lawyer has got a court order requiring MNHQ to disclose the details held about the poster.
That’s all. This isn’t some important ruling, it is a court order to disclose information. It isn’t a test case. The lawyer has not “won” - MMHQ didn’t contest the application, presumably because (as is their stated policy) the poster did not require them to.
Why didn’t the poster want them to contest the application? Because they had given false details on registration, so have not much to fear from them being disclosed.
Even if the poster had used genuine details, the lawyer would still need to bring a legal claim for defamation - only if they won that could they genuinely claim a victory.
Oh, and on the wider point - comparing laws in the UK to countries that don’t respect human rights is fanciful.