LONDON—JPMorgan is challenging British rivals on their home turf with the launch on Tuesday of its long-awaited digital retail bank, Chase, as part of what the U.S. lender hopes will be a global expansion. The launch marks the first foray into retail banking outside North America by one of the United States’ most dominant lenders, heaping pressure on British incumbents such as Lloyds, Barclays, NatWest and HSBC which are already battling low interest rates and upstart digital rivals. “We have been watching in which markets customers are really ready to do their banking primarily through digital channels, and the UK frankly leads the way in this respect” said Sanoke Viswanathan, chief executive of the new Chase bank venture. The venture, if successful, could see the U.S. bank expand into continental Europe and then globally, he said. “This is a business that we are building not just for the UK but …