Sorry, let me be clear.
I am in the UK.
I have set *my* preferences to UK.
When I share my details with friends in the UK - I want them to see UK MPG.
I don't want the entire site to change. I don't want every user in the world to see UK MPG for all cars. I just want viewers of my cars to see my MPG in my local units.
I think fuelly is set to US MPG by default, so if your friends in the UK want to see your results in UK MPG format, then they will have to set it themselves in the top left corner of the screen.