Because, on top of one of 'em, there's a great big phased-array radar, pointing right down to Saudi Arabia. Check out this paper by Pavel Podvig of RussianForces, and scroll right to the bottom for the radar coverage map. Alternatively, there's this Globalsecurity.org page, which is rather out of date. But the key point is that the Russians have a large phased-array radar down in Gabala, Azerbaijan, with enough range to cover the vast majority of Iranian territory, Iraq, the Gulf, and northern Saudi Arabia. The only bit of Iran it can't see is the hardscrabble desert down on the Pakistani border, a long way from anywhere. On the other hand, everywhere in that part of the world near anywhere is covered.
Meanwhile, is anyone concerned about some details regarding the Trident decision? Consider the White Paper, and specifically the possible cases it gives. Three options are given - one is the obvious one of buying the next US SLBM system, another is the silly one of a homegrown ICBM system, and another suggests the procurement of a force of large airliners and the development of a new very long-range cruise missile.
Neatly, the stated alternatives are all knockout arguments for the favoured option. Option 2, a helpful graphic explains, would require missiles to be placed in every corner of the nation (subtext: will affect house prices in your constituency) in order to match the dispersal provided by a submarine patrol area. It would of course also require inventing a brand-new huge rocket. Option 3 is blatantly silly, indeed dishonest.
The RAF is in the process of buying some new aeroplanes - specifically, the Eurofighter Typhoon's later variants are intended to be the best strike fighters in the world. They are capable of carrying the Anglo-French Stormshadow cruise missile, with a range of 200 miles. If it was given a nuclear warhead, and all the options assume that AWRE Aldermaston would develop one, that would be enough to attack Moscow with a degree of certainty. But inventing a whole new cruise missile, one with enough range to be launched from a large civilian aircraft, which is what Option 3 assumes - well, that's going to be absurdly expensive. After all, an airliner-as-bomber would have to launch well away from any possible air defence, unlike a Typhoon or for that matter an F-35.
Note the classic bureaucratic technique. Pre-filtering means that the choice presented to mere democrats is kept down to a choice between the impossible and the expedient. And what is the problem with the cheapest and most independent option? Silence...