Hi nick,
Iran is your best route but unless you are a dual citizen of a non UK country you will have to have a guide which will be expensive and constraining depending on the individual you get.iranians are wonderfully friendly people and if you are prepared to throw cash at it the issue of a guide could be a positive one.
Also check with the bikers on the thread who did it without guide as it is no suprise to me that there are ways around this.
The carnet for your vehicle will not be a lot.over the years i have driven from Dubai to Uk three times and each time i have arranged with a work colleague or friend to return the carnet to the ATC and for them to return the money to me.You can nominate a returnee to the ATC for this purpose at the time of collecting the carnet.however,please check this as systems there tend to change.The ATC is best visited first thing in the morning before they get disintereted.They are very helpfull.
A Turkish visa can be obtained easily at the border.Regarding a route thro Turkey try and check and definately continue to update the situation in Eastern Turkey with the Istanbul bikers club, if they see this thread they will probably contribute.Be prepared to change your plans at the last minute.All routes thro are great experiences.
Good luck and happy trails,
Tony.