This is the UK we're talking about and it's nearly October. You'd struggle to power a flashlight from solar power at this time of year.
Edit: at least in the UK, they only claim green, not fully renewable.
You do raise a good point - as electricity is fungible, and the power grid is built, operated, and maintained by a national institution, then how can a particular energy provider promise that their power is more renewable than that from the the others? Otherwise, every time Bulb gets a new customer, we are supposed to believe that they get all the renewable electrons, and the power supplied to the rest of us is just a bit more fossil-fueled.
I do know the answer, and that is that those companies just buy and sell the various "pinky promise" certificates and accreditations that pass for being "renewable" and "carbon-neutral" , and have nothing at all to do with actually boosting the supply of renewables.
The UK is also suffering from a fuel shortage that is affecting electricity distribution, exacerbated by a heavy reliance on wind and an ongoing period of low winds.
Oddly enough, it's all the little "100% renewable" power companies that are going bust, when you'd think they were independent of natural gas prices.