When one of them wandered out alone for an after-hours stroll around town, he was stopped by a police officer, who bundled him into a taxi and sent him back to his hotel.
Still, they could hardly complain of being bored in their lodgings: wireless internet was available throughout the trip.
But when they asked for the password to access it, they were told: “It’s your passport number”.
The authorities monitor web traffic and do not even bother to hide it.
It is mostly a big political story that is dominating Iran-related headlines at the moment.
The government in Tehran announced last month that it would stop complying with more of its commitments from the nuclear agreement it signed in 2015 with the United States and five other countries, including Britain.
The big commitment is uranium enrichment: Iran says it will violate a part of the deal, which sets out how much of the element it can stock, as early as next week.
Israel and the US, now under Donald Trump, are adamant this is part of a plan to build a bomb and the US president reintroduced economic sanctions last year.
The agreement itself still stands until Tehran officially breaches it, but a European diplomatic push to keep Iran onside seems doomed to fail.
As if that were not enough, there is the very real threat of war. Last week, two oil tankers in the Gulf of Oman were damaged by sudden explosions.
The US says it was caused by a limpet mine attached to the hull of at least one and argues that only Iran has such sophisticated technology.
Most independent analysts agree: it could be a sign of Iran deliberately showing what it is capable of in the seas to its south, through which 40 per cent of the world’s internationally-traded crude oil passes.
Tehran denies involvement in the explosions. But on Monday, the Pentagon announced plans to deploy 1,000 more troops to the Middle East.
Mr Trump and his team seem to believe this and their economic sanctions, which affect ordinary Iranians the most, will help rattle the 40-year-old regime of the Ayatollahs.
There is no doubt the sanctions are biting. Trade with western countries is collapsing and Iran is experiencing inflation, recession and a black market trade in foreign currency, all the signs of an economy under external pressure.
My friends saw the depreciation first hand: the official rate is 47,230 rials to the euro but they found their money would buy three times that in Iranian notes.
Lunch for their group of nine people at upmarket hotels would never exceed 1.5 million rials. Officially, that should equate to €31.80, but the lower street rate meant it cost them just €10 (£8.90) for the entire meal.
A less powerful Iranian rial means that anything imported — electronics, medicine, even specific foods — becomes more expensive.
There is rising discontent and mass demonstrations are fairly routine, but it is still a far call to say — as Mr Trump has done — that this is a government is on verge of collapse.
Britain, which supports the 2015 nuclear deal, is uncomfortable with talk of military options and has worked with European countries to try and salvage an arrangement.
But the fact is that the UK has less of an incentive than others to oppose US economic sanctions: its trade interests lie elsewhere.
France exported five times as many goods to Iran as Britain did last year, according to a report by the Bicom think tank, while German exports were 14 times higher.
UK goods and services are far closely tied to Arab countries that oppose Iran’s influence in the Middle East.
Given the ongoing detention of UK citizens in Iran, including Nazanin Zaghari-Ratcliffe, there are no attractive options for the UK. We are set for a long, hot summer in the Strait of Hormuz.
