"I, for one, would like the same applied to the Iranian authorities here."
I don't think that the Iranians expelled anyone, we chose to reduce our diplomatic presence there. Our plan was that that would make life awkward and there would be internal pressure to improve relations with the UK.
The "same" would be the Iranians choosing to withdraw from the UK, which they're unlikely to do.
If we actually expel their diplomats then that is not the "same" and is the sort of thing that is going to escalate into a nasty tit-for-tat type thing.
The Iranian Government would probably quite welcome that happening as then they can say "see they have no wish to be diplomatic with us, their solution to our problems with them is clearly going to be military!"
We need diplomatic relations with Iran right now more than they need them with us, we're desperately trying to resolve the nuclear issue.
"Childish yes but how else do you respond to a country like that."
And that is the quandary that the west is in right now, likewise with North Korea which is notably higher up the scale of evil than Iran, but still below Chris de Burgh.