UK Petrol price

So i just bought petrol (Gas for the cousins across the Atlantic) for the first time in a year and paid £6.91 per gallon, about $9.34. I fuelled what was my wife's Ford Focus 1600. Looking at the receipt I bought 19.73 litres for £30.17 or $40.79. Here's the kicker. That purchase just added ONLY 1