Our works out at $10.27 for gas, and $10.65 for diesel so the price difference is similar.

Not on percentage basis. I think people are less likely to quibble about 40 cents when you get into that $10 range. Down in the $3 to $4 it is a bigger jump. With UK pricing being per liter, the difference posted on the sign drops into the 10 to 15 cent range. Which can be less than the difference between regular octane and mid-grade here. The sticker shock in thus less.

Then a surprising number of people don't actually notice the price after the decimal. That's why marketing something that costs 9.99 as under 10 works. To them gas is $3 and diesel $4, which is a 33% increase.