i was a few months in 2002-2003 in kazakhstan, kirghistan, ouzbekistan, turkmenistan, iran, pakistan ... i know those countries little bit, maybe more :-)
i did not dare going to tajikistan and afghanistan and i wouldn t recommend it also. i had the afghan visa and a special un convoy (i worked a bit for the unfpa pakistan).
even pakistan, check before, nowdays, it doesn t sound to good (well, i m back in paris, not there now ...), especially if you want to travel free and safe of course.
check my website (vincent.danna.free.fr) especially this link
try any embassy in the region for your iran visa, i managed to get it in kirghistan, try to get your embassy, the un (if you know anybody there) involved ... but yes it s different for british-us citizens.
the french embassy has helped me a lot on the way.
some backpackers go there (afghan and tajik) but they are with local transport, with local people, people are really nice there but if you meet the wrong one (?)
if i remember well, after my decision not to go to afghan, an italian biker went there and got killed (check the history and details on this site).
up to you ...
i also know norbert who has been free without paying in china recently, check this link
he may help you