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jake-the-peg, the political reaction would be sheer amazement, the UK does not have an independent nuclear capability. Although we are buying Trident from the Americans, they will not release the firing codes that trigger the weapon, even the Brits, their closest allies are not trusted not to turn on them.
We would not need to launch a nuclear attack on them, when you consider that we have troops in Iraq (handily right next door) and in Afghanistan, (quite close by), American troops and, lets not forget the regions bogeyman Israel but let us not consider the Israelis yet, an attack by conventional means would be far more probable. However, such an attack would be a political disaster for the West, it would give the Islamic fundamentalists the reason to call a Jihad (holy war) against the West.
Militarily we could beat the Iranians, it took them 8 years to contain the Iraqis, and they were 3rd rate.