Thames Valley area
Just FYI: Thames water covers a larger population than that. Much of London is not technically in the Thames valley.
EDIT: I'll add. It's anecdotal, as I can't remember when or where I read it. But I seem to remember a Thames water complaint a good while back. That much of the older London area they cover has not been worked on for over 100 years. And is listed such that the cost is beyond them.
Not sure if I buy it given modern techniques etc. But I'm no expert.

Huge saving. LOL, a bit like saying don't worry your only going to die of a bullet, we save the rockets for the big boys.