LONDON, Jan 26 (Reuters) - Britain on Sunday said it would force retailers to implement stricter age-verification checks to stop children buying knives, in a tightening of policy days after a teenager admitted killing three young girls at a Taylor Swift-themed dance event. Axel Rudakubana's knife attack in July was described last week as one of the most harrowing moments in Britain's history by Prime Minister Keir Starmer and has triggered a public inquiry into the failings that allowed it to happen. While that inquiry is expected to focus on why state institutions failed to act on warnings about the...