Land(ish) based?
Just a thought, which will almost certainly be pooh-poohed, but still worthy of brief consideration:
In light of the fact that the UK has horrendous debts, and really can't afford to do anything expensive, new delivery mechanisms for these weapons are more likely to succeed politically if they are cheaper.
How about putting the missiles in land-based silos? The missiles have a 7000km range, I believe, so that's enough to reach pretty much anyone that might feasibly need a nuclear spanking from the UK. No need to target any of the Americas, I don't see any of those states becoming nuclear-armed and belligerent during the remaining life of the Trident D5. Surely land based silos are much cheaper than nuclear submarines. The even cheaper option would be to just park the existing boats in dockyards and fire them from there, if technically feasible.
Of course they would be vulnerable to sabotage or a surprise first strike, but then so is a submarine to some extent. If they are geographically dispersed, an enemy would need to take all of them out simultaneously. The Russians, Americans, Indians, Pakistanis and Chinese all have land-based long-range missiles. Even though all of those countries are much larger than the UK (and hence can hide the missiles somewhat better), UK has a fairly wide spread of sovereign territory around the world that most of the others don't have.
The UK's atomic weapons are now of a strategic nature only, and either will trigger a massive retaliation if used, or are themselves a massive retaliation. Once the missiles in the submarine(s) currently on patrol have been launched, that's yer lot. It's not like they'll tootle off to King's Bay to be re-armed. They are therefore a deterrent weapon. Would a potential aggressor really be able to wipe out say 4 locations simultaneously? Hell of a gamble to make, both ways. Fail to take them all out and you will be annihilated. Fail to prevent them all being taken out, and the UK will be annihilated.
But annihilation is pretty much guaranteed anyway.