I just had a flight from LHR with Iran Air, and it went really well. The personnel was courteous and the check-in procedure was quick. Although security lines were a little slow, the Iran Air LHR Terminal itself was well-organized. The lounge had some cozy sitting and was passable. The flight itself was enjoyable, and the boarding was punctual. Overall, you can anticipate a hassle-free trip with ordinary airport amenities if you're traveling Iran Air from LHR.