How badly have you screwed up...

if Iran can kidnap 15 British soldiers, and still have the moral high ground?

Here's the thing: releasing those letters of confession, videotaped apologies, etc. is a war crime. Prisoners of war aren't supposed to be used for publicity, as instruments of propaganda etc. etc.

So why aren't the British (or the US) screaming "War Crimes!" to high heaven? Because the Iranians would laugh, and would compare, say, three letters from soldiers who say they've been well-treated for a week to show trials of people who were held in secret prisons and tortured for five years.

And the US and its allies can't really look good in that comparison.
That, by the way, is a pizza ad.