Just pasted this from above, sorry I should have quoted their post with my question re visas.
Vinvertebrate · Today 18:28
Sorry @Westfacing - I posted a fulsome (waffly) explanation, then trimmed it down and it didn’t make sense.
Basically, UK/US/Canada citizens can only travel to Iran with a visa, which is rarely granted. (Theoretically, you need to be with an organised group and have a local guide accompanying you at all times, and you might also need to bribe an official because they don’t like handing visas out to the likes of us - we are infidels of Little Satan/Big Satan (in the regime’s eyes) so why would they?
The vast, vast majority of Westerners visiting Iran WILL be dual nationals (like my DH), even if they have lived outside Iran not speaking Farsi for decades. So saying “not dual nationals” doesn’t really make sense because normally they’re the only ones that CAN visit.