The actual amount is disputed. As I understand it, the shah ordered and paid in advance for 1500 tanks (plus some other military stuff) from a shady MoD-related arms company called IMS. We dutifully started manufacturing the tanks, then the Islamic revolution happened and we decided not to hand over a load of military hardware to the hardline nutters who wanted to eliminate the West, Israel, etc. 🤷♀️
Iran has been pissed off ever since. There was a court case in the early 90’s and it resulted in about £300 million being paid into court by the UK to meet its liability. Iran couldn’t actually get its hands on that money, so remained pissed off.
Nazanin’s “ransom” of £400m was probably considered an end to the matter from the UK gov’s perspective, but apparently not for Iran. If you can extract money from a country by holding citizens to ransom and you are a bunch of terrorist nut jobs anyway, why wouldn’t you?
fwiw DH is dual nationality UK and Iranian and he won’t set foot in Iran these days. Way too dangerous in his view (although people still do it ofc).