Things change: Attack on Britain was playing to popular resentment of Britain & perhaps avoiding confronting USA's renewed appeal among many Iranians.

From The New York Times:

When Ayatollah Ali Khamenei used his speech at Friday Prayer in Tehran to denounce Britain as “the most evil” of Iran’s enemies, he was striking a chord with a deep resonance in the psyche of Iranians, the legacy of a long history of British imperial intrusions into their country’s affairs.

Singling out Britain, and not the “great Satan” of the United States, so often the bugaboo for Iran’s leadership since the Islamic Revolution of 1979, might seem an odd choice for Iran’s supreme leader, when the government he leads faces its greatest crisis in 30 years.

But British scholars on Iran said Ayatollah Khamenei’s attack on Britain was characteristic of a Tehran leadership that resorts under pressure to a mix of crude stereotypes that play well at home. They said it might also reflect a concern not to slam the door opened by President Obama, who is offering a new dialogue in his search for a diplomatic solution to the impasse over Iran’s nuclear program.

If that were the calculation, Ayatollah Khamenei may have correctly concluded that going after Britain would cost Iran little, judging by the carefully hedged response to the Tehran speech by Britain’s prime minister, Gordon Brown.