I haven't read all the replies,
but this from Wikipedia says:
Mumsnet is a London-based internet forum, created in 2000 by Justine Roberts for discussion among people with child- or teen-aged offspring.
History and finances
Mumsnet was created in 2000 by Justine Roberts, who came up with the idea of a website to help parents pool information and advice, following a disastrous first family holiday with her one-year-old twins. Roberts persuaded friends Carrie Longton and Steven Cassidy to help her build the site.
The website grew to become one of the most influential online forums for parents; in November 2009, then–Prime Minister Gordon Brown, opposition leader David Cameron and many other leading UK government ministers took part in live webchats with Mumsnet users.
In March 2010 Mumsnet's 10th birthday party was hosted by Google UK at their London headquarters. Guests included Ed Miliband and Steve Hilton, and both then-Prime Minister Gordon Brown and his wife Sarah Brown gave speeches. Gordon Brown referred to Mumsnet as a "great British institution".
In May 2011, Roberts founded Gransnet, a sister site to Mumsnet for users over 50.
Roberts, the CEO, was named in The Guardian's 2010 Power 100. In February 2013, Roberts and co-founder Longton were assessed as the 7th most powerful women in the United Kingdom by Woman's Hour on BBC Radio 4. Roberts was appointed Commander of the Order of the British Empire (CBE) in the 2017 New Year Honours, for services to the economy.
In 2018, Mumsnet had 1.3 billion page views from 119 million unique users, and a revenue of £8.6 million.
In April 2020, Mumsnet announced a premium membership option.
so based on the above, it was initially London, England based and has grown...