It occurs me that there is a face-saving manouevre that could be used if the power-that-be decided that some kind of accomodation was needed to avoid trouble.

No need to declare the poll invalid or anything like that - they just need to decide that, in view of a need to avoid divisiveness, the two leading candidates should go on to a runoff election, as would have been the case if the one who cane first had not crossed the 50% level. A maganimous gesture rather than a climb-down...
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Interesting that Khameini has focussed his anger on the British rather than on the USA. I would be inclined to read that as being in the nature of a coded conciliatory gesture to "the Great Satan", as the USA has been generally described for the last few decades.