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.
Having a UK passport does not really offer this couple any protection. The regime doesn’t give a fuck about the publicity and might want to set an example. Their only thing they have going for them is that the regime and the UK have an ongoing scuffle over debt owed by the UK, giving them some currency. I think a death sentence is unlikely, but not impossible. If they’d sneaked in and kept their heads down I suspect the outlook would be better, but every single idiotic thing they have done whilst in Iran says “spy” if you are a paranoid fundamentalist who hates the West.