Which route are you taking?
If you are going Europe-Turkey-Iran-Pakistan-India-Nepal then over to SE Asia then you don't need any 'letters of invitation'.
What you do need is a Carnet de Passage from Iran on and it is easiest to get your visas for Iran and Pakistan before you leave - you need to have a valid visa for your next destination after Iran before they will issue you a visa.
Contact the Iranian embassy in London to see what the requirements are for your visa - I went direct to them rather than through an agency and they were quite helpfull if a tad slow in issuing the visa.
Pak visa easy to get, Indian visa can be obtained in Islamabad, Nepalese you can get on the border, Thai is visa on arrival then you can get any other visas you require for SE Asia in Bangkok.
Choose Life. Choose a job. Choose a career. Choose a family. Choose a ****ing big television.
Choose sitting on that couch watching mind-numbing,spirit-crushing game shows,
stuffing ****ing junk food into your mouth.
Choose rotting away at the end of it all, pishing your last in a miserable home,
nothing more than an embarrasment to the selfish, ****ed up brats you spawned to replace yourself.
Choose your future. Choose life.