1) No missile over flights over nations you have no quarrel with.
Who gives a flying f***? It'll be an orbital object - or good as - so there are no national airspace considerations by international law. Anyone who does object will merely be tacking it on to their horror at you nuking an Iranian target in what will be seen as an overwhelmingly disproportionate response - turning Iran from the aggressor to the victim.

2) Leaves no doubt which state you are after
It's called a BALLISTIC missile. That means it can't manouver more than a tiny amount, and is on a ballistic track to the target pretty much from when it is launched. Anyone tracking it for two or more points on it's route can make a pretty accurate prediction of where it is going to land. Launching from the Persian gulf is actually likely to make India, Pakistan and China more nervous as the submarine will be within range of them and any missiles would be heading their way. From UK territorial waters it would go nowhere near Russia or Israel and would be clearly in it's terminal stage over Iran.

3) Shorter flight times for time sensitive targets.
Iran has no form of BMEWS system. Thus, the first warning they would have would be when the missiles were in their terminal stage, at the very best - and their C4I systems are so primitive that they would be unable to get the warning to people in time to move them. Beyond that, I really can't think of any nuclear target where the difference between 10 and 30 minutes would be critical - particularly considering that it would take hours between a launch order being given by the Prime Minister and the weapons hitting in reality. There is no way launch authority would be devolved to commanders on the ground in such a scenario, and this effectively rules out use on time sensitive targets.
All that is necessary for the triumph of New Labour is for good men to say nothing whilst CGS.