LONDON—David Cameron, the former British Prime Minister, said on Thursday he was not motivated by his own financial interest when he lobbied the government on behalf of the now-collapsed supply chain finance firm Greensill Capital. Although Cameron’s extensive lobbying efforts did not result in policy changes, they have raised questions about the extent to which former British leaders can or should use their status to try to influence government policy. Cameron declined to say how much he was paid by Greensill, or how much he would have made from shares he owned if the business had prospered, but said he had “a big economic investment” in its future and wanted the business to succeed. “I was paid an annual amount, a generous annual amount, far more than I earned as prime minister,” he told the House of Commons’ Treasury Committee. When he left office in 2016, Cameron was entitled to …