Does your fiancee have a Ukrainian passport also?
Is your studio flat big enough for a couple and have a double bed?
Whether you apply for a spouse visa or fiancee visa is up to you but you will.need to show that you meet the requirements for finances and accommodation. You will need to show that you earn a salary of at least £18,600.00 and have been at least for 6 months.
If granted the fiancee visa, you will have to marry within 6 months and then apply for a spouse visa.
The alternative is marrying in Ukraine or Iran and then applying for a spouse visa.
I hope this answers your question. If so, kindly rate the answer and provide feedback.
If you have any further questions, please do not hesitate to ask.
If she marries in Iran and then applies, it makes no difference. I cannot comment on whether the marriage would be deemed valid in Ukraine but I cannot see why not if it is legally valid in Iran.
It can take up to 6 months. Same for a spouse visa
I hope this answers your questions. If so, kindly rate the answer and provide feedback.