LONDON—British Prime Minister Boris Johnson personally covered the cost of refurbishing his Downing Street flat, a minister said, seeking to quash claims by Johnson’s former top adviser that the prime minister had planned to get donors to pay for it secretly. “What I know is the prime minister has personally met the costs of the flat refurbishment and that is what people in Britain want to know,” Liz Truss, Britain’s trade minister, told BBC television on Sunday. Dominic Cummings, who was Johnson’s key adviser on the Brexit campaign and helped him to win an election in 2019 before a bust-up last year, said on Friday that Johnson wanted donors to secretly pay for the renovation and he told the prime minister the plans were “unethical, foolish, possibly illegal”. Johnson’s office has said the government had followed the rules over the refurbishment. But the opposition Labour Party is calling for an …