Firstly we could extend the service of our current 4 subs like the USA did, which would save around £5 billion. Another option is to reduce the number of warheads (approx 160), as you say a lot of the cost is R&D however running costs are significant also and would be reduced by reducing the number of weapons or reducing the number of subs to 3 (as has already been suggested), it would also help reduce the 2-3 billion refurbishment cost. Another option is not to use a submarine based system.
While I can see the first point I'd be interested to see how much that would save, considering the new subs would most likely be extended Astute subs, thus not costing a huge amount more than the reconditioning, plus having the added advantage of running longer than the refurbed subs.
Warhead reduction would depend on a couple of things, one, how many subs we have and how much time eah warhead takes to be serviced. We need at least two times the number of warheads to the overall capacity of the subs, possibly 3 times. That is to make sure all subs are full to capacity incase they are all needed at short notice and also to make sure we have enough fully serviced warheads to fill them (therefore 2x minimum, however I have no idea ho long they take to service hence the possibility of 3 times). I'd be fine with losing any excess warheads after all that has been taken into account (but I can't see it being many).
With regards to reducing the number of subs. The BARE MINIMUM is 3 subs to have any hope of a decent deterrent (any less and we may as well not have any), one on service, one in short term refit and one in long term refit. Ideally we would want four at least, so you have more leeway if refits take longer/accidents occur and so you can have a couple out at sea and changing over easier. How many do we have now? Four, for that very reason.
Finally your suggestion of using another system. There are really only two choices there, aircraft or missile silos. We tried aircraft and it hasn't really worked however if we did decide to go with them we would have to develop a nuclear bomber, probably on a par with the B-2 stealth bomber the US have, or we may as well not have any as there would be no suprise and we'd be shot down (obviously if we managed to get any off the ground in the first place). Price? Probably similar or more than the submarine option, and certainly not as good. Second option is silos. Aside from the fact they would probably take a decade at least to build (probably more due to local complaints) they would cost an absolute fortune and no one would want to live anywhere near them. Again they are very susceptable to being destroyed before they can launch any missiles as their locations would be known almost immediately...
With our current technology submarines are probably the safest, cheapest and best way of providing a nuclear deterrent. How are you going to destroy one when you don't know where it is?
Why do we even need 160 nuclear warheads? Are we intending to take out an entire hemisphere or something?
I'll admit I'm not particularly clued up when it comes to defence, but cutting that number would surely still be a sufficient deterrent?
Because warheads need to be serviced too, so need redundancies. I guess you also have the need to have excess just incase the first wave wasn't enough...*gulp*
Big numbers, mind = blown
