It’ll only be a third because of defence cuts. Frankly £100 bln over twenty five years would yield an extra £4 bln a year which in defence terms is tiny. Also you forget that we’re playing with the big boys in defence terms: conventional forces not backed up by a nuclear deterrent will last hours in the next war with Russia because the Russians will simply use tactical nuclear weapons.
It’s a useless white elephant provided that the other side agrees to fight with one hand tied around its bollocks and we agree to do the same. I’m not saying that it couldn’t happen, but it was the threat of using chemical weapons that stopped the Germans using chemical weapons in WW2 and vice versa: Churchill would have used mustard gas on German civilians but for fear of German retaliation, which is a fucking good job because the Germans had secretly developed nerve agents. If one side had renounced chemical weapons the other side would have used them.
In every war there is a temptation to use the most powerful weapons and that temptation is only tempered by the costs of retaliation by the enemy with those same weapons. So your military strategy is dependent on the goodwill and cooperation of the enemy: you expect Putin to get more of his own people killed and make war harder on Russia rather than choose the easy option of just hitting any NATO C4ISTAR, POL site, airfield, choke point etc with tactical nuclear weapons in the opening hours of the conflict. Or you expect that there will never again be a major war in human history. One of the two: either there will never again be a major war or that the enemy will be nice to us in that war.
The moment you accept the possibility of a another major war then you are forced to accept a country that possesses nuclear weapons will be tempted to use them. If you accept that they will be tempted to use them then you accept that only the threat of retaliation with nuclear weapons will prevent them from using them. Again I reiterate: There is historical precedent for this: This is the scenario that played out in WW2 with chemical weapons. Consider the scenario that played out in WW1 also: one side thought it had chemical weapons and used them in the belief that only they had them and that they therefore had a strategic advantage. Sadly it didn’t turn out this way and the war ended up being more horrific than it needed to be.
Now which scenario are you most willing to live with? The mutual deterrence scenario of WW2 or the scenario of WW1 where one side’s belief that it alone possessed WMDs lead to the use of WMDs? Personally I’ll live with the former even if it costs £100 bln, that’s quite a cheap price to not get nuked because the total value of all the schools, hospitals, etc not to mention all the lives would be lost, the geopolitical-economic costs of losing a major war are vastly less more that £100 bln.