The primary cause of this current price differential is the enormous taxation applied to diesel and petrol in the UK. Obviously this situation won't last.
My main point is that even with happy hour prices this is only viable for people which do not have to rely on external chargers, which is the majority. Currently EV is cheaper again, due to rapid rise of fuel prices, but the power companies with street side chargers generally "adjust" prices every year to match petrol and it's been for a long time cheaper to run petrol. Also: getting petrol from a few km away for cheap is easy, you are not going to park in the next village overnight to charge your car.