According to David Cannadine's review in the Timesonline, Porter's argument seems to be that ordinary Britons knew very little about the overseas Empire even during its heyday. Also, Porter claims that even when there was a fair amount of awareness, the fact of the Empire had little concrete effect on British politics. In some sense this is a polemical book against a certain kind of postcolonial scholarship, which argues that "Empire was so pervasive that the Victorians didn't need to talk about it; or "It's all the more pronounced because it's absent." Porter is right to be critical of those kinds of arguments (I am critical of them too).
Though my knowledge about 19th century England is somewhat limited, there may be some truth in what he's saying. The 1857 Mutiny, for instance, initially caused only a small ripple in England, considering how devastating it was in India. Parliament got around to responding to it, but not for a few months (though part of this delay is certainly due to the slow dissemination of information at the time). No governments fell, no heads rolled (a little like the non-impact of the Iraq debacle in US politics, actually). The real lasting effect of the event would be seen within the colonial government in India itself, where a series of new laws would be passed in the following decade that codified, for the first time in 'black letter' law, a penal code, marriage laws, and even laws about religious conversion. (Many of which are still in practice today, believe it or not... but that's a grumble for another day)
Still, I gather that Porter's point is that very few governments rose or fell on the question of Empire. One other small objection/question: what about Disraeli in 1870? Labour ran against him, charging that he was an "Imperialist"? Meanwhile, he proudly asserted his "imperial" credentials for the first time (earlier he had been tetchy on Empire), as part of an obviously opportunistic turn to populism.
Also interesting in this particular review is the litany of dates and events that Porter is apparently saying are unimportant.
And when Joseph Chamberlain sought to enlist the nation to join him in his great imperial crusade of Tariff Reform, the Conservative party was roundly defeated in the general election of 1906. For much of their lives, Chamberlain and his small group of co-imperial zealots — among them Lords Curzon and Milner, Rudyard Kipling and Leopold Amery — despaired of rousing the British to a full appreciation of their imperial reach and global responsibilities. Neither the Diamond Jubilee of 1897 nor the Imperial Exhibition at Wembley in 1924 made any lasting impact.
Porter apparently is in the awkward position of arguing that something obviously huge in the historical sense was actually unimportant to ordinary people. But why, ultimately is that important? And how can we be quite sure? And this is Cannadine's main objection to the book:
One of the hardest things for a historian to do is to try to demonstrate that something was not important. And unimportant compared to what? It may be true that most 19th- and 20th-century Britons did not know much about their empire; but it is not clear that they knew much about anything else. Where does that get us? Despite the book's subtitle, there is little discussion of Scotland or Ireland, and, as the author coyly admits, his conclusions might be very different if he had extended his discussion to encompass those two nations where empire did bulk larger in the popular consciousness. And even if his argument is correct, it might still be the case that there was more "imperial culture" in Britain than in, say, imperial France or Spain or Russia or Austria-Hungary.
The reviewer, David Cannadine, is himself by no means a raving leftist-postcolonialist. In 2002 he wrote a book called Ornamentalism: How the British Saw Their Empire, where he argued that the hierarchy of imperial authority had more to do with class than race. So this is not an ideological hit. His objections to Porter above seem quite reasonable.
Still, I plan to have a look-see at this book when it comes out next month.