Upvote:1
You have to obtain a UK visa. You can apply for a visa in the United States, using the following link:
Upvote:13
Iranian nationals need a visa to visit the UK, even if holding a US green card.
To visit friends in the UK, you need a Standard Visitor visa. You can apply online for the visa.
After you apply online, since you are in the US, you will give biometrics at a USCIS Application Support Center. There are many offices all over the US, so you should not have to travel far to reach one.
After obtaining your biometric confirmation receipt, you will post this, along with your passport, visa application form, supporting documents, and prepaid return envelope to have the documents returned to you, to VFS in New York at the address given in the instructions.