From China to Iran

Hello!

I am confused....I am in Tibet and I would like to travel by land through Kyrgyztan or Tajikistan/Uzbekistan/Turkmenistan to Iran, and am a UK national. It appears that it is impossible to cross from Turkmenistan to Iran, is this so?

And for which of these countries would I require a letter of invitation? I am not so seasoned, but would prefer to keep things simple and work my way independently by land. My definite goal is to make it to Iran, for where I will arrange a visa in China. My other route would be through India/Pakistan or India and then by ship to the UAE then by ferry to Iran. This route is off-topic, but I have to get there somehow! And I would also appreciate any advice about moving from Pakistan over into Iran without getting killed..unlikely I know, but..

Any info greatly appreciated, about visa/LOI requirements and so on.

Thanks,
Darius