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Since you fled from Iran you will lose your refugee status if you go back there to get married to her. So she has to come to you instead.
Here is a very useful website provided by the British government that gives you an overview on how she can apply for a "Marriage Visitor Visa":
You can click through the pages to read about the eligibility criteria and requirements she has to fulfil.
Note that she has to return to Iran after a maximum of 6 months. Before that happens you can apply for family reunion. Please check out this link and read the information given under the headline "Apply in the UK":
In a nutshell you both have to be in the UK to do family reunion, it has to be her first application and you need to prove that your relationship already started in your home country before you were getting persecuted.
Probably it's best for you to get in contact with local counselling centers in Great Britain before you start the procedures outlined above. They are more familiar and experienced with the legal framework and can probably give you more detailed advice than me.
I hope this answer was helpful to you.