Diplomats working at the Iranian embassy in London must leave Britain by Friday afternoon.
Foreign Secretary William Hague ordered their expulsion within 48 hours on Wednesday afternoon after the British embassy in Tehran was stormed. They are expected to fly out this afternoon.
About 120,000 people in the UK hail from Iran, as well as many students.
The Iranian Association's Kaveh Kalantari said most UK-based Iranians "do not like what is going on in Iran".
He said they were "worried".
The diplomats are expected to leave the UK on a charter flight out of London later this afternoon, according to BBC Persian TV.
Tuesday's attack by hundreds of protesters followed Britain's decision to impose further sanctions on Iran over its nuclear programme.
Deputy Prime Minister Nick Clegg told the BBC's Sarah Rainsford the UK's relationship with Iran had "taken a very serious knock".
He said: "Something really bad happened when the Iranian authorities allowed those people to overrun our embassy compounds, and it is quite right that we have been very clear in our response - as have many other European countries who have withdrawn their ambassadors for consultations.
"It doesn't mean we're cutting off all diplomatic relations with Iran. It doesn't mean we are in any way lessening our determination to try to find a diplomatic solution to the nuclear question, which is immensely important to Europe and the whole world, and we will continue to work tirelessly to find a negotiated solution."
Mr Kalantari said of Tuesday's attack by protesters: "I was really depressed when I heard that these stooges of the government had occupied the embassy."
He told the BBC he believed the embassy occupation was part of a "power struggle" between different factions in Tehran but he added: "Iranians here in Britain are worried about the bad press we are getting."
The sanctions led to Iran's parliament reducing diplomatic ties with the UK.
"If any country makes it impossible for us to operate on their soil, they cannot expect to have a functioning embassy here," Mr Hague told MPs on Wednesday.
The foreign secretary said there had been "some degree of regime consent" in the attacks on the embassy and on another UK diplomatic compound in Tehran.
He also said all UK diplomatic staff in Tehran had been evacuated and the embassy closed.
Mr Hague said relations between the UK and Iran were now at their lowest level, but the UK was not severing relations with Tehran entirely.
Iran's foreign ministry described the British move as "hasty" and said Iran would take "further appropriate action", Reuters news agency quoted state TV as saying.
Germany, France and the Netherlands announced on Wednesday that they were recalling their ambassadors to Tehran for consultation, and Norway said it was temporarily closing its embassy there as a precaution.
Hundreds of protesters - whom Iran described as "students" - massed outside the embassy compound on Tuesday afternoon before scaling the walls and the gates, burning British flags and a car.
Another UK diplomatic compound in northern Tehran, known locally as Qolhak Garden, was also overrun and damaged.
Iran said it regretted the incident, which it described as "unacceptable behaviour by a small number of protesters".
Mr Hague said the majority of those taking part had been members of a regime-backed Basij militia group.
He said the private quarters of staff and the ambassador had been ransacked, the main embassy office set on fire and personal possessions belonging to UK diplomats stolen.
The US, EU and UN Security Council also condemned the attacks.
Last week, the US, Canada and the UK announced new sanctions against Iran, including measures to restrict the activities of the Iranian central bank.
The UK said then it was severing all financial ties with Iran.
The move followed a report by the UN's nuclear watchdog that said Iran had carried out tests "relevant to the development of a nuclear device".
Iran denies the accusations, saying its nuclear programme is solely for peaceful purposes.
On Sunday, Iran's parliament voted by a large majority to downgrade diplomatic relations with the UK in response to the recent action.