If I am an Iranian passport holder and have a green card. Do you need a visa for UK?

Yes, you still need a visa. But it's better if you apply from the US not Iran. If you are an Iranian green card holder, you can visit Canada without visa though. Same with Bahamas, Costa Rica, Bermuda and Jamaica.
* Remember to always have your passport with you when visiting these countries otherwise the US won't allow you to comeback with just the green card.