The Foreign and Commonwealth Office (FCO) advise against all travel to:
within 100km of the entire Iran/Afghanistan border
within 10km of the entire Iran/Iraq border
the province of Sistan-Baluchistan
the area east of the line running from Bam to Jask, including Bam
The protests that were widespread across Iran in late December 2017 and early January 2018 largely ceased as of mid-January. However, there remains a risk that these protests may re-occur or new demonstrations begin on other grounds. You should avoid any demonstrations and follow the instructions of the local police.
Terrorists are very likely to try to carry out attacks in Iran. Attacks could happen anywhere, including in places visited by foreigners. On 7 June 2017, there were 2 attacks in Tehran – one in the Majles (the Iranian parliament) in central Tehran and another at the Imam Khomeini Shrine near Imam Khomeini international airport. See Terrorism
Iran is a Muslim country in which Islamic law is strictly enforced. You should respect local traditions, customs, laws and religions at all times and be aware of your actions to ensure that they do not offend. See Local laws and customs
British nationals need a visa to travel to Iran. You should make your visa application well in advance of your date of travel. The application process for an Iranian visa can be long and unpredictable. The Iranian Embassy has told the Foreign and Commonwealth Office that to get a visa you must either apply as part of an organised tour, or have a sponsor in Iran. See Entry requirements
Take out comprehensive travel and medical insurance before you travel.