Re: "MAD has been proven to be cobblers in Ukraine"
"... well, if Ukraine had kept it's "share" of ex-Soviet nukes, you might be right. But since Ukraine isn't a nuclear armed state, one can hardly say that its current difficulties are due to a failure of MAD, however else you might describe them."
Au contraire, the point being made was that the UK and US guaranteed Ukrainian sovereignty. When the civil war broke out in Ukraine and Russia made a land grab, despite their guarantees and their nuclear arsenals, the UK and US decided to do nothing - they weren't going to risk a war to defend the crooks in Kiev. It is precisely because the UK, US and Russia have nuclear weapons that the Russians (correctly) surmised they would not end up in a major international conflict. Even if Ukraine had nuclear weapons, would that have stopped Russia? I doubt it - either through forcible neutralisation, or by a calculated gamble that Kiev don't want to die.
So I come back to the issue, where's the value to the UK in a full fat submarine launched deterrent? In particular, the underlying principle of MAD was detente, which is only possible if you have a full suite of escalating conventional military and tactical nuclear options before your strategic deterrent. Our conventional military forces cannot field a single aircraft carrier, have no naval surface forces above the scale of a destroyer, comprise a bare handful of antiquated Tornado strike aircraft and some new build but ancient design concept Typhoons. Our army has a tiny handful of attack helicopters and collectively the military have trivial numbers of transport helos and transport aircraft (of which the newest are currently grounded). The army has been reduced to a size where the Horseguards outside Buck House are probably imposters employed by Crapita, or carrying Equity cards.
I think we should retain nuclear strike capabilities. But we don't have the capabilities necessary to support MAD via detente (that only ever really worked against a major, imposing enemy, rather than numerous, diverse, rapidly changing threats), and we can't afford to replace Trident. So it seems that we need to cut our cloth according to our circumstances and yet still offer our military the equipment it needs to do the duties we ask of them. I simply don't see a new submarine launched ICBM system as being a good "investment", compared to a cruise launched system that could be deployed in a greater number of submarines, launched from ships, silos or air-launched. And that would leave a lot of money for conventional military kit, rather than disappearing into the coffers of large defence contractors.