Well, different ideas about the same reality
As dear Arasp Said, Tehran is not a good idea if you are thinking about making a good money, if you are serious about living in Tehran, you should have other intentions, like knowing Iranian culture or learning Farsi.
Again as Dear Arasp said, Being a native teacher is a privilege, but first you should be a good teacher and I guess you are.
I don't agree with dear Arasp about British Accent, it is not a matter here, since IELTS Certification is very popular here, so your British accent can be even useful.
I don't agree with dear Arasp about expenses either, the pension which I mentioned ( 200$ per month) was in Hamshahri newspaper just a weak ago, and already I know some students who are living in similar conditions,
With all respect, Online training in Iran has just started, your first goal as I mentioned before should be working in British council as an English teacher, just if you can achieve score 9 in IELTS, I guess you can!
Finally I believe Iran doesn't have stable economic condition, but it is not as unpredictable as Dear Arasp explained for you
Well, I see you are making some new friends, even before you get here! It is brilliant! I'll be happy to meet you too, perhaps we can arrange a meeting when you arrive.
Don't worry about Tehran, just continue researching about it.
Regards
Babak