OK back to the original question.
As the coalition forces in Iraq are there to provide security at the request of the Iraqi Interim Government (I know you can argue how true that is but I do believe it to be agreed international legal position) then anyone aprehended and found guilty of an offence against said forces would be subjected to the full force of the Iraqi legal system including the death penalty. If they turned out to be British (from whatever religious group or ethnic root) they would still face the legally handed down penalty and the FCO would take normal consular action to try and get the sentence reduced.
Of course if repatriated to UK following stalwart work by the FCO they might strugle to make a living in Aldershot, Colchester, Salisbury, Caterick ........... well anywhere really.