Believe it or not, I was visiting Skytrax (a very good site to visit) this evening and they have opinions from pax who have flown in virtually every airline you can think of, from ACES to Yemenia. Iran Air got a pretty lukewarm response. Food was well rated, but that was about it. Aircraft were (for obvious reasons) old and a bit decrepit and inflight entertainment was bad - an old projector showing movies in farsi. Farcical, more like!
However, one writer mentioned connecting at Mehrabad and gave it a particularly emphatic thumbs down. Now, you want to go to NRT
. Tuesday is the only day on which flights connect, i.e. leave LHR
on Tuesday at 17.00, arrive Tehran at 02.00, leave at 19.45 the same day and arrive NRT
I'd really recommend you try someone else! There are plenty of decent European airlines around - SAS, for example: shiny new A340s, PTVs, STAR alliance points. And you REALLY would chose IR