£100 to fill up, so what, put less in, cost less!
I think quite a few drivers are doing this, and then running out of fuel. Putting less in each time only means visiting the pump more often to put less in, so if they're not reducing the number of miles they drive, the only saving is the fractional increase in fuel economy gained by carrying less weight in the car. Perhaps drivers need to put less in, and run out more often, in order to wean themselves off driving everywhere. Perhaps little machines like the Honda Super Cub that do 200mpg are surely what people need if their commutes and errands are longer than what's reasonable for cycling, and if they can't afford to relocate to live closer to amenities.