Why does Iran hate Britain?
They blame us (and the Russians) for the Persian famine of 1917-1919 when some two million (out of a population of 10M) died. Britain and Russia occupied the country at the time. More information here.
They remember that we (and the Americans) emgineered a coup in 1953. This overthrew the democratically elected government of Mohammad Mosaddegh and restored the rule of the shah. More information here.
Earlier this month, British Royal Marines helped detain an Iranian tanker in waters off the British overseas territory of Gibraltar, after Gibraltar’s government said it believed the tanker was transporting oil to Syria in violation of EU sanctions. More information here.
Actually, many Iranians – especially in the cities where support for the regime is weakest – are well-disposed towards the British (and other Western nations) as I found when I visited the country in 2009. An account of my trip here.