[ben_w]

As it only takes one nuke, my assumption is that anyone with nukes and ICBMs can take out a power grid or grids spanning any region the size of the continental USA.

On that basis, yes, you could also take out the Russian or Chinese grids this way.

North Korea could also perform such an attack.

I assume the UK can too; although I don't know the maximum altitude of a Trident SLBM, the maximum range suggests it could well be great enough for this mode of attack.

Non-nuclear EMPs do exist, but you should probably ignore them at this point unless you're actively working in that field, given all the easier solutions in EW including simple jamming.