In reply to Stefan Jacobsen:
> You can get a tourist visa on arrival. They have an office issuing them at the airport. Works for Danes, don't know about Brexiteers
Incorrect. UK, USA and Canadian passport holders need to apply via an agency before they go.
OP: There is a very lively and useful FB group called See you in Iran. It would be worth a look, but your agent should be able to inform you of everything.
The Foreign Office has significantly tightened up security since the Daesh attack in Tehran in June.
I can highly recommend someone here for more info:
http://persianpursuits.com