I've just applied for an Iranian tourist visa at the Consulate in London. Bear in mind that visa regulations are fluid so the following is my experience as of March.
1. Get an authorisation number via an agency. I got mine within 10 days from Iranian agency Uppersia http://www.uppersia.com. Complete the online form then email a scanned copy of your passport details. After about 10 days hopefully you will receive your authorisation number.
2. Once the authorisation number is obtained, allow 3 or 4 working days before applying for a visa at the embassy. This allows time for the number to be forwarded from Iran to your chosen visa application centre.
For Iran visa check: http://www.uppersia.com/Iran-travel-services/iran-visa.html
Check the cheapest Iran tours at http://www.uppersia.com/Iran-tours