I think the ~20% is how much it should take up if people are behaving rationally (and can also be an exaggeration, so it wasn’t intended to be something mathematically rigorous). The current regulatory environment and decades of divesting in fission construction coupled with continued investment in wind and solar makes those options look more attractive than they are economically and politically. Also important to note that there are regional differences that impact the cost benefit for a given type of tech and I don’t know if the UK has a particular geographic strength around wind like California does with solar.

There’s simply not enough battery capacity and physical land in the world for solar power plants and wind turbines to provide all the energy. Everyone always does the solar calculations ignoring the fact that energy has to be produced semi locally to where it’s used (long distance transmission is expensive and lossy). Additionally the calculations for energy capacity of renewables ignores the fact that energy that the capacity can’t be shifted to match load (without batteries).

Seriously. This is why the fossil fuel industry loves solar and wind. They’re not a serious enough threat to their business.