TOKYO—Duncan Scott spoke of the “Adam Peaty effect” after becoming Britain’s joint most decorated Olympic swimmer of all time on Friday with a silver in the men’s 200-metre individual medley. The medal was his third of the Tokyo Games, after 4 x 200-metre relay gold and 200-metre freestyle silver, and fifth from two Olympics. The Scot won two relay silvers at Rio de Janeiro in 2016 and is not done yet. Britain has won six medals so far in the Tokyo pool, matching the country’s best performance since the 1908 Games, and Scott can become the first British athlete from any sport to win four in a single Olympics when he swims in Sunday’s 4 x 100-metre medley relay. “This isn’t just a one-meet thing for British Swimming, it’s been going on for a while,” he said. “It probably started with Adam Peaty back in 2015 when he first won …