You're making out that doesn't have to happen now. When demand spikes Dinorwig starts up, water is pumped back up between 3 and 5am. Dinorwig is not used for peak hours, just spikes. No extra energy would have to be produced than is now to have that facility. That's a real world example already in use, of a green way of coping with spikes.
At peak hours extra energy could be produced by bringing more renewable sources online. Obviously renewables are not as flexible as fossil fuel power stations, if the wind aint blowing you aint got the energy. So, the question is, is it logistically possible to run a grid purely on renewables? Many experts think so, many don't. I'm not convinced by either argument, but even I'm less convinced by the nuclear argument. I think it's the politicians easy, lazy way out. It's passing a problem on to the next lot, after they're dead and gone.