Although both Blair and Brown are technically anti death penalty generally anyway, i think the issue was the decorum of the killing itself.
For all he may have deserved Much much worse etc - and probably deserved no respect at all, whatsoever - there are two main points:
One is practical, in terms of Internationsal opinion and Sunni sensibilities. Given rape and murder and the abuses of prisoners seen by the International community, it reflects badly on the US and also therefore on the UK , but mainly it reflects Very badly on the Iraqi government:
it says the Iraqi government is Weak and pitiful, that it cannot even control it's own guards and executioners -
i.e. : it's a free for all for all insurgents: the Gov't is a joke, and weak and corrupt -
The second is not so much about any given individual who is hanged, but about us as a Society.
I beleive even the scummiest twats killed in the states are given a last meal etc and the actual killing is done with a certain amount of respect and decorum, no matter how evil they are.
At that point it is literally seconds away from being in Gods' hand -
it should be done cleanly and professionally and without such shenanigans.
