Khamenei knows that this is a no-risk gambit. The Obama administration has made it abundantly clear that Britain is no longer a valued ally. The Iranians are quite sure that the Brits won't stand up for themselves given the UK's craven response to the kidnapping of their sailors. So why not:

[Khamenei] said the election was a "political earthquake" for Iran's enemies - singling out Britain as "the most evil of them" - whom he accused of trying to foment unrest in the country. "Some of our enemies in different parts of the world intended to depict this absolute victory, this definitive victory, as a doubtful victory," the supreme leader said. The UK government summoned the Iranian ambassador to protest against the ayatollah's remarks, although the embassy sent a more junior diplomat in his place.

The junior diplomat they deigned to send was the charge d'affaires, presumably because the ambassador couldn't be bothered.

When Khomeini's thugs were rampaging through Tehran during the Carter administration, they had to make a choice between attacking the US embassy and/or attacking the Soviet embassy. The Islamic Revolution was ostensibly aimed equally against both countries. Khomeini regularly railed against how Iran had become a site for Cold War intrigue: tirades against Communism were a rhetorical staple and his "prophesies" about the fall of the Soviet Union are still a common feature of Iranian state propaganda.

But for some reason only the US embassy was overrun. The Soviet embassy was never attacked. Almost as if weakness begets contempt.

References:
* Four Theories Why Obama Is Throwing Britain Under The Bus [MR]
* British Impotence [MR]
* Ayatollah demands end to protests [BBC]
* UK summons Iran's ambassador over Khamenei speech [AP]

Previously:
* Jimmy Carter: Iran Is Nothing To Worry About
* Britain Down On UK-Israeli Ties, Pretty Psyched About Nuke-Building Terrorist Supporting Syrian Regime
* Britain Clearing Dozens Iranians To Study Nuclear Engineering At British Universities. Yes, Really. (Plus: Europe Won't Go Along With US Sanctions?)