LONDON (Reuters) -Britain’s antitrust watchdog said on Friday it had formally launched an investigation into the $19 billion merger between Vodafone’s UK operation and CK Hutchison’s Three UK, reviewing whether the deal will hurt competition. The tie-up, announced last year, will reduce the number of mobile networks in Britain from four to three, challenging a tenet long held by regulators that four operators in major markets helps keep prices low. The Competition and Markets Authority (CMA) now has 40 working days to investigate, or until March 22, by which time it must decide whether to prog…