HI, here is a very helpful Iran climate graph. Make sure you scroll down a little and look at 'high temp' and not just 'mean temp' (which averages the min and max). Some places are not quite as hot as you'd expect (though some are).
http://www.climate-charts.com/Countries/Iran.html
Yes it will be hot, but if you just accept the fact you will sweat like mad each day and therefore don't take too many good clothes along, and take plenty of sunscreen and use it, then you will be okay. (It makes your evening showers just that much more satisfying!)
Yes the heat is dryer and therefore better than in England, but when it's 38-43 each day knowing that it's slightly less humid than your used to doesn't help all that much.
In my humble opinion, if you can get over the heat it'll still be good. Your wife and you will toughen up for a few weeks. Yes you will sweat, yes you will dose up on sunblock 2-3 times a day, yes you will down 5 litres of water a day, but who cares it'll still be good.

