I think a mix of air-launch and sub-launch nuke cruise missiles would give the flexibility and not cost anything like the Trident boats and their infrastructure? The re-furbished Tridents on Diego Garcia idea bears some thought too.
I am afraid you think wrongly. Your proposals would be more expensive to give the same capability, or inadequately capable for the same cost. Against people who we really need to persuade not to try the nuclear blackmail game then pretty much only SSBN armed with Trident (or equivalent) is adequate. And that requires 4 SSBN to give lowest through life cost. Whilst the politicians may willfully think that going to 3 boats will be cheaper as is usual they will be wrong.
Consider the following scenarios:
1. You detect an enemy launch against Diego Garcia. You have only minutes to launch in retaliation before you lose all your silos. Use them or lose them. What do you do. Ooops, it was a false launch warning. Pity you just launched.
2. You launch a conventional cruise missile against a terrorist target somewhere. Another country somewhere near the destination thinks they are the target and it is nuclear armed. They launch their nuclear tipped IRBMs against the UK. You apologise for their error of judgement. Oh, and against serious opponents the loss rate and time to target of cruise missiles is inadequate, especially as we are trying to reduce the number of warheads.
3. You'd like to insert some SBS on a surveillance mission. The only SSN in the area is partly armed with nuclear tipped cruise missiles. You can't seriously risk losing the entire nuclear deterrent in shallow water just to get a few recon troops on land.
4. etc etc etc (you actually need more not less nuclear infrastructure with all your ideas)
No, when you look at the real issues, 4 x SSBN is the only thing that gives a true second strike capability anywhere reasonably needed during the lifetime of the system at minimal cost.