Thames Water has picked the US investment firm KKR to take a stake in the business, as the embattled water company fights to stave off nationalisation. The UK’s biggest water supplier, which is struggling under a debt pile of close to £20bn, said it had selected KKR as a “preferred partner” as it seeks to secure fresh equity funding for its operations by the end of June. The New York-based private equity firm is expected to acquire a stake in Thames worth £4bn. The UK-based business water retailer Castle, with more than 250,000 customers, had also put in a bid