Whenever you see a problem with unit conversion like this it's probably because there's a setting off somewhere. Keep in mind that Fuelly has two important unit settings. One setting for display units, and one setting for reporting units. The display units set what you see when you view all vehicles across the site. You set this value by choosing US, UK, or Metric under the logo on any page.
You choose reporting units when you set up your vehicle. You can decided whether to report your fuel-ups in Litres or US gallons. You currently have your vehicle set to report in US gallons. That's probably not what you want. Since you're in the UK, you buy in Litres and can simply type in that number if you have your vehicle set to report in Litres. If you change your vehicle reporting units to Litres instead of US gallons you should be set.