Yet, poverty is a complex and contested subject in Britain, where there are several contrasting measures of deprivation, and some observers were skeptical about the necessity of Mr. Alston’s visit.

While child poverty has risen markedly in recent years, overall poverty levels have remained fairly steady or even dropped slightly — partly because of sharp falls in pensioner poverty since the 1980s. Though 1,550,000 were classed in 2017 as destitute, or experiencing extreme poverty, by the Joseph Rowntree Foundation, a poverty research group, that figure was a quarter less than in 2015.

If Mr. Alston ends up linking poverty to the government’s benefit policies, “he would just be adding more noise,” said Edward Davies, director of policy at the Center for Social Justice, a think tank founded by Iain Duncan Smith, the Conservative minister who spearheaded the reforms. “My fear is that we have a debate about welfare reform in this country, and he could get embroiled in that.”

Commenting on Mr. Alston’s visit in a written statement, a government spokesman said that child and overall poverty rates had fallen since the Conservatives entered office — using a different measure from the one preferred by poverty researchers at the Rowntree Foundation and the Child Poverty Action Group.

Mr. Alston said he was keeping an open mind. In several meetings in Newcastle, he avoided asking leading questions and frequently played devil’s advocate to people seeking to blame the government.

As his train thundered north into Scotland that night, Mr. Alston said he would draw no conclusions until releasing his preliminary findings on Friday.

“It’s been said to me, ‘Come on, you must have had a draft before you came here,’ ” Mr. Alston said. “But on every mission I’ve always found myself asking after a few days, ‘What the hell am I going to say?’ ”