I have led several tours to Iran. It is an incredible country to visit and I have never had any problems with taking photographs. The usual caveats would apply such as anything military-looking or strategic such as borders etc. (I did nearly get into trouble for taking pictures of the old US Embassy in Tehran but managed to talk my way out of it)
As has been said, the FCO has a country-wide travel advisory at the moment so if you did go then your insurance may be void. Also, there is no Iranian Embassy operating in the UK currently so you would have to get a visa from Paris or Dublin. All do-able for the determined and if you did go then you would find nothing but urbane, extremely hospitable people eager to show you their wonderful country.
If travelling independently, you may visa support from an Iranian travel company. One that I have used is
www.welcometoiran.com - they also offer their own tours.
I have a hunch that diplomatic relations will be restored between UK and Iran fairly soon. If so, just GO - you will not regret it!
You can travel around the country freely using local buses as I have done (assuming that you are not a US citizen and also male!) It is not impossible as a female but can sometimes prove a bit of a problem using public transport.
I have a link to some of the images I took on my first visit.
http://www.photoboxgallery.com/imagesoftravel/collection?album_id=94099210Hope this helps a bit.
David