Hello Emily @flyinglampost
Welcome to the Wefugees community and thank you for your question.
1) No, it's not possible to apply for asylum in another country that signed the Dublin III convention. The UK will check back with the EURODAC database and find out that he already claimed asylum in Austria before. Hence he will be sent back.
2) In general it should be possible to marry in Austria if you can provide the required documents. Get in contact with the competent registry office ("Standesamt") and ask them about the details. Both of you will most likely have to hand in a valid passport, a certificate of birth, an official confirmation of residence ("Meldebestätigung") and a certificate of no impediment that proves that both of you are eligible to marry. At least that's what I found out through some research on the internet.
In general, foreign documents that aren't written in German need to be translated by a verified translator. You need to submit both the translation and the original document. Maybe there are special agreements between Austria and Iran regarding acceptance of each others documents. The officials at registry office can tell you more about that.
Regarding your wish to live together in the UK:
There is a quite useful website provided by the British government where you find extensive information on how to settle in the UK permanently:
First of all, your partner needs to get a "partner of a settled person" visa. For a married person it's valid for 2,5 years. After this time he will need to apply to extend his stay. Please check out the following link to get more detailed insights:
After having lived in the UK for 5 or 10 years with such a visa he can apply for a permanent residence permit (a so-called "indefinite leave to remain"). As far as I can see the duration depends on the requirements he needs to fulfil (e.g. there are no financial requirements if he choses the 10-year route). Here you find the details:
Since we are mostly focussing on questions about the German asylum system etc. it might be a good idea to get in touch with British organisations that are more familiar with the legal framework overseas. As a start, here are two links that you can check out: