[dragonwriter]

> If say South Africa has done away with nuclear weapons capabilities, then why can't UK?

South Africa only did it because the right-wing white minority regime knew it was falling to internal opposition that was not only racially different but perceived to be of diametrically opposed ideological and, at least potentially, geopolitical orientation, and didn't want to supply nuclear arms to a hostile group. I suppose if the British government expected to fall to, say, a domestic Chinese-aligned quasi-Maoist opposition and had sufficient lead time to do so, it might disarm, too.

If you don't know why most nations don't voluntarily disarm in any other circumstances, the example of one of the few that did—Ukraine—might clear that up.