This from Richard Beedall's Web site:
The government identified a number of options for future of the UK's strategic nuclear deterrent, in approximate order of increasing cost these were:
•Don't replace Trident - leaving France as the only EU member with nuclear weapons;
•A small force of vulnerable land based ballistic missiles.
•Ship based ballistic missiles - probably initially Trident.
•Build a new class of SSBN's, initially carrying extended life Trident missiles or a replacement submarine ballistic missile system.
•Develop a cruise missile with a nuclear warhead, air launched by a force of 20 RAF strike aircraft supported by 20 dedicated refuelling aircraft.
A submarine based solution was announced as the preferred choice in December 2006. Initial government estimates were that the programme would cost about £20 billion, perhaps about £9 billion of which (in current money) is for the replacement of the replacing the four Vanguard’s with another class of 16 tube ballistic missile submarines. If this money was taken solely from the RN's current share of the MOD's equipment budget then no other new ships or submarines could be ordered for the RN for a period of at least 15 years.
SLEP'ing the Vanguard's
In addition to new build, MOD officials investigated the costs involved with a service life extension programme (SLEP) for the four existing Vanguard class submarines, and their Trident missiles, re-entry vehicles and W-76 derivative nuclear warheads. Since 2006 the government and MOD have apparently assumed as a near given a 5 year extension in the V's service life, from 25 to 30 years.
Developments in the USA (see below) indicates that it might be possible for the UK to economically extend the system to serve for yet another 10-15 years - but this would be very costly. At the very least the boats would unexpectedly need another expensive long overhaul period and refuelling (LOP(R)), and their hulls and nuclear steam plant would need to be critically inspected and recertified. Some UK experts (e.g. Commodore Tim Hare RN (Rtd)) believe that the Vanguard's are being too hard worked for a full SLEP (extending their service life by 20 years) to be possible, rather than the 5 year life extension widely discussed. it's also worth noting that SLEP'ing the V's would almost inevitably result in the loss of a national nuclear submarine construction capability, and this will have to be factored in.
Across the Atlantic, the US Department of Defense is pressing ahead with a Trident II D-5 life extension (LE) program, the first contracts for which were awarded in 2002. The USN now expects to operate the Trident II D5 missile from SLEP'ed Ohio class submarines until about 2042. Also, on 26 May 2005 the US Department of Defense announced the award to Lockheed Martin a $9 million contract in regards to development of an affordable Submarine Launched Intermediate Range Ballistic Missile (SLIRBM). The SLIRBM will have a maximum diameter of 32.5 inches, less than half the diameter of a Trident II, but a range not exceeding 2000 nm, again less than half that of a Trident II. The missile could accommodate both nuclear and non-nuclear GPS guided warheads. Allegedly, UK MOD officials have already been involved in discussions about the project.
http://frn.beedall.com/fsm.htm