I bought Mum.co.uk to strengthen my hand with Mum.com. My logic was Mum is a British English word so it has a nice fit with .co.uk and mum related sites are heavily developed on .co.uk.
As far as I know Edwin originally owned Mum.co.uk with other family related .co.uk generics like Dad.co.uk, Kids.co.uk, and Kid.co.uk. He auctioned Mum.co.uk at Sedo in 2007, bidding reached £8,000 versus a reserve price of £15,000.
I believe Edwin then sold a portfolio of domains, including Mum.co.uk and Dad.co.uk, to Domain Invest in Luxembourg. I assume they wanted to sell Mum.co.uk because it didn't generate enough traffic.
Domain Invest raised 10m Euros from venture capitalists in October 2009. Their biggest success to date was selling Voodoo.com for $300,000 in February 2009 during Mardi Gras in New Orleans, having paid $31,000 for it a few week earlier.