From The Foreign & Commonwealth Office website:
"The FCO advise against all travel to the whole country.
The Foreign and Commonwealth Office (FCO) advise against all travel to Iran. British nationals have been arbitrarily detained in Iran in 2010 and 2011, and there is a continued risk of this occurring again.
If you choose to travel to Iran against this advice you are strongly advised to keep a low profile, stay away from any demonstrations or gatherings, avoid border areas and limit your stay to the minimum period possible. See Safety and security.
During a demonstration on 29 November 2011 the British Embassy in Tehran was attacked and set on fire. The Embassy is now closed.
The Embassy of Sweden has assumed the role of protecting power for British interests. If you need urgent consular assistance you should contact the Embassy of Sweden in Tehran. See Consular assistance.
Dual nationality is not recognised in Iran. The Iranian authorities will deny that the British government has any legitimate responsibility for British Iranians and is unlikely to grant any consular access.
There is a general threat from terrorism. See Terrorism
Take out comprehensive travel and medical insurance before you travel."
Your travel insurance WILL NOT BE VALID if you travel here, as nearly all insurers, my company included, will not pay out if you have travelled to a country against the advice of the FCOS.