Imran Farooq, 50, was a founding member of the Muttahida Qaumi Movement (MQM) but had moved to London and had not been active in the party for about two years. He was on his way home from work in north London when he was attacked.
The three accused were members of the party. Their motives were unclear.
Besides their life sentences, the three were each fined 1 million Pakistani rupees ($6,050), the prosecutor, Khawaja Mohammad Imtiaz, told reporters.
Britain gave Pakistan legal assistance, sharing evidence and having its officers testify at the trial, according Toby Cadman, counsel to a Pakistani government barrister, Toby Cadman.
Imtiaz said it was first trial of its kind in Pakistan. "There are hardly any examples in the world that an offence is committed in one country and the trial is conducted in another," he said.
Farooq's death marked the start of cracks in the leadership of the MQM. The ethnic party, which held sway for decades in Pakistan's commercial capital, Karachi, later split into several factions.The party represents Urdu-speaking migrants from India who settled in Pakistan after the partition of the subcontinent at the end of British rule in 1947. ($1 = 165.3000 Pakistani rupees)