LONDON — In the final days of 2020, the U.S. health insurer Centene made a swift incursion into Britain’s prized National Health Service, one of the world’s largest employers. A Centene subsidiary, Operose Health, took over nearly three dozen medical practices in London — gateways for NHS care — in a deal worth tens of millions of dollars. The subsidiary became the largest private supplier of general practice services in the United Kingdom, with 67 practices accounting for 570,000 patients. A local health commission, records show, signed off after a nine-minute review in a virtual hearing held…