British Nationality could be obtained through your birth, or through the birth of any one of your parents or grandparents, in Iran. This arises because of Britain's relationship with Iran and Britain's Colonial history.
To determine what form of British Nationality you might have, an analysis of your family's history must be done, and then applying it to the Nationality laws of Iran that have been passed over the years.
STATUS OF IRAN
From 04.03.1857 until 09.05.1928, the Crown exercised extra-territorial jurisdiction over Iran.
Iran is now considered a foreign country for the purposes of British Nationality.