I know petrol cars have improved mpg in recent times but I remember years gone by when we used to drive down to the Costa Brava Spain every year. As company cars I had a Ford Focus Ghia 1600 petrol in 2006 then in 2009 I had Ford Focus Titanium 1600 diesel both estates fully loaded with roof boxes and bike rack on the back. With the petrol the trick was to get to the Spanish border on three tanks, the fuel is/was significantly cheaper in Spain, with the diesel it was two tanks.
That is easy to achieve getting to Spain on one tankful because it’s all down hill but coming back to the UK it’s all up hill. ?