It's the tight coupling between the government and arms manufacturers. I used to work at UKTrade and nearly every decision there was rooted in how we can give more money to arms companies.

It's really an agreement to work towards disarmament, not to just unilaterally disarm regardless of what everyone else is doing.

Article VI: Each party "undertakes to pursue negotiations in good faith on effective measures relating to cessation of the nuclear arms race at an early date and to nuclear disarmament, and on a Treaty on general and complete disarmament under strict and effective international control".

I can't see how for example Russia could be disarmed under strict and effective international control in the current climate.

We're not increasing the number of our warheads in service, just replacing old ones with a new design.

No they haven't. Did you even read what that treaty is about? Countries that already have nuclear weapons get to keep having nuclear weapons, and countries that don't have nuclear weapons will agree not to develop them in exchange for receiving peaceful nuclear technology. Aside from that, countries must "pursue" disarmament, but the treaty does not actually require it to happen.

Given the recent stirrings of Russia, and our proximity to their main path to the Atlantic (where most of the world's undersea internet fibres are), disarming would be incredibly foolish.

Given recent trends over the pond, it's probably good to have our main deterrent independent from Washington.