I think problem is that the numbers you have entered at Fuelly do not match the numbers you're showing me here. I think you need to edit your fuel-ups so that the number of litres you're telling me here match the number of litres you see on the site. Once you do that, the correct MPG should follow.
OH, I think I understand now! You have your vehicle set to enter fuel volume in gallons, not litres. Yes, for fuel entry, we only support US gallons. It's our understanding that in the UK you get numbers in litres at the pump, right? Are you converting from litres to UK gallons in order to post to Fuelly?
Sure, sorry about the roundabout way of getting to this point. You'll probably need to go in and edit those fuel-ups to reflect the amount in litres instead of gallons. Once the reporting units are set for a vehicle, the system assumes all fuel-ups are reported that way.
The cleanest way to start over is creating a new car. Just keep that in mind if problems crop up.