Yes, folks in the UK use a different gallon than folks in the US. The UK gallon is slightly bigger at 4.546 Litres. The US gallon is 3.79 Litres. So "Miles per Gallon" is going to mean slightly different things to those two groups.
According to your profile you're in the UK, so you probably want to use UK MPG exclusively so you're comparing apples with apples when you talk fuel economy with other UK folks.