The Economist's columnist Bagehot last week surmised:

FOR the English, there were many reasons why losing the American colonies was annoying. One was that America had been a handy place to exile convicts, some 40,000 of them over the years. George III took a personal interest in the hunt for new spots to resettle those (in his words) “unworthy to remain in this island”. Gibraltar was considered, as was west Africa, before ministers plumped for newly discovered Australia. By the time transportation ended (accused of lowering the tone of the Australian colonies), almost 200,000 men, women and children had been shipped Down Under, most never to return. Transportation was sorely missed: Parliament pondered new penal colonies in the Falkland Islands and even Antarctica, amid public panic at the idea of ex-prisoners roaming English streets.

Repeatedly in history, when faced with rising crime or mob violence, respectable English citizens have yearned for those who alarm them to vanish: whether via the gallows, by removal to the edges of the earth, behind prison walls or (after race riots in the late 20th century) through calls for immigration to be curbed or reversed.[...]

However, he concludes, history has caught up. Now there is no alternative to mounting a pragmatic project to make British society once more work. But I am not sure that David Cameron sees it that way.

Perhaps there are other options. Prison hulks moored in the Thames perhaps?