Been mentioned many times that the readout is not accurate.
Best way to check is to use actual useage of fuel to miles covered.
But to do accurately is to brim the tank, use, then brim again.
You don't need to use all just a fair amount of fuel and a fair miles.
However, depending on type to driving, urban, rural etc will have outcome on mpg.
But need a few fill ups to get best average mpg results.
Also, the car has to "learn" your driving style.
It gather info as you drive but to quicken this up you may wish to clear the ecu.
You will find post on here about clearing the ecu by disconnecting the battery for 30 minutes.
Re uk or usa mpg
unless usa import then would expect uk mpg