First, some history. Disraeli coined the idea of one nation in 1845, a time of the Chartist uprising and when memories of the French revolution were still vivid. "One nation" Toryism was motivated by a desire to fend off such a revolution. As Disraeli said in the Free Trade Hall speech cited by Miliband, he saw the purpose of the Tory party as being to maintain the constitution and prevent revolution. He thought the way to do this was to have a benevolent hierarchy - responsible capitalism - in which workers can improve their well-being with the parameters of existing power relations. Bourgeois socialism, said Marx and Engels. is "desirous of redressing social grievances in order to secure the continued existence of bourgeois society."
"One nation Toryism" was an attempt to accommodate working class power. It's no accident therefore that it has gained influence when workers were powerful or militant (the 1840s and post-1945 years) and lost influence when capital regained the whip hand.
With workers now quiescent, the social pressure towards one nationism is missing.
This matters because there are at least four psychological mechanisms which bias today's rich against benevolence:
If meritocrats believe, as more and more of them are encouraged to, that their advancement comes from their own merits, they can feel they deserve whatever they can get.
Lord Grantham, who owes his fortune to the luck of inheritance, might feel a sense of noblesse oblige. But bankers don't.
2. The just world effect. People have always had ways of convincing themselves that inequalities are in fact justifiable, that they deserve their fortune and the poor deserve theirs; "He made them high and lowly and ordered their estate." But in Disraeli's time this was accompanied by a Christian compassion which is weaker today.
3.The abundance effect. Mere proximity to cold hard cash encourages selfishness. It's no accident that, historically, noblesse oblige has been associated more with those whose wealth is in land than with cash-holding merchants and bankers
These four mechanisms are consistent with Jason Marsh's claims that the rich are less likely to be empathatic or compassionate than others.
All this suggests that there is neither the social nor cultural basis for the sympathy between the classes which Disraeli thought necessary for One Nation. And given the power of companies over the state, we should be sceptical of how far One Nation be can created by legislation alone.
If you want One Nation, you need a more powerful working class. The rich and powerful might not be motivated much by benevolence - but they are by fear.