[iam-TJ]

The change-over was actually driven by the technology at the time - namely the outdoor road-facing price advertising panels, as well as many of the pump displays.

Simply - they were only designed to show 2 numbers so when the price-per-gallon exceeded 99 pence many had to stick a static "1" in front, and many point-of-sale terminals and cash registers couldn't handle it.

Since there are ~4.5 litres in a British gallon displaying pence-per-litre brought the displayed prices back to 2 digits and allowed for a gradual transition to 3 and 4 digit displays.

And yes, at the time the switch from pence-per-gallon to pence-per-litre occurred some retailers did take advantage to 'add some profit margin' but it wasn't universal.