We've held off on doing imperial gallons because fuel is no longer sold in those units (and hasn't been for some time, correct?). Our UK testers early on said that everyone has converted to L when filling up and 100km/L was the figure most often used around economy, just like the rest of the world, so we went with that.
Even though I'm in Canada (Eh!) and record information in Litres, I still convert to imperial gallons and show myself both mpg and km per litre. For us older folk who still go back and forth between metric and imperial, it would be nice to have that option.