Trying to work out how many nuclear weapons needed to destroy the UK is a bit of a 'how long is a piece of string' question but here goes anyway. I'm assuming the data uses American Tons of 2000lbs or 907kg. It doesn't matter that much in terms of the discussion.Worldwide there are approx. 30,000 warheads with a total capacity of about 5thousand million ton of TNT [5000megatons]. So that gives each warhead about 170,000 ton or roughly eight times the 22000 tonnes of the bomb used on Nagasaki. [The biggest ever detonated was 50megaton & 100MT is believed to be enough to cause a Nuclear Winter].
The Nagasaki/Hiroshima bombs each devastated about 5sq miles. The area of London is about 620 sq miles & Birmingham about 100 sq miles. Again without defining what is included & what is not these data are very approximate. However we might say, without getting too tied up in irrelevant accuracy, that 10 average bombs on London, two on Birmingham, 3 on Manchester would effectively render those cities completely disfunctional. It doesn't take us long to realise that maybe 30 would effectively wipe out the UK.
Doesn't really matter whether the answer is 20 or 50. In this ball park an enemy like the ex Soviet Union could reduce us to nothing with only our Trident Submarines still functioning. So we could hit back. Yippee. Big Deal. But the Soviet Union isn't there any more. Only the USA has this capacity in terms of warheads & delivery system. Is Liam Fox seriously considering a need to deter an attack from the USA?
Would a terrorist or a rogue state with the warheads & capacity to deliver two or three bombs on the UK [only Israel can do this right now] be deterred by our Tridents? That number of bombs would still leave us functional & deterrence/retaliation could very effectively be from Cruise missiles.
Liam Fox made some good points in his speech to the Tory Party conference but his affirmation of a replacement for Trident, as a sop to the Tory 'Right', is what should truly differentiate the LibDems from the Tories on Defence. Trident is a completely indefensible waste of money. On every possible argument, & there are many besides the one I have just made here, it has no value whatsoever.