By Sarah Young and Marissa DavisonLONDON, March 5 (Reuters) - In an area of London known as Little Tehran, the killing of Ayatollah Ali Khamenei prompted spontaneous street parties, dancing and fireworks. Shoppers still smile as some stores hand out sweet pastries, traditional in Persian culture when there's cause for celebration.But as the bombardment of their homeland enters its sixth day, the joy is beginning to fade.