As with any such reports I'd take the HJ figures with a pinch of salt. They list the Mii and Citigo with wildly different MPGs, for example, something which shouldn't possibly be able to happen if there were any control/scrutiny of the methods used and figures being supplied.
There are too many factors that influence MPG, but one thing seems certain; the Up can get pretty much the lowest consumption of any petrol car out there.
Whether or not you or I actually achieve that is another matter.
For instance, I live in a very hilly area so almost as soon as I leave the house I have a 200m hill to climb. With a cold engine this is always going to hammer the MPG, especially if there are a lot of short journeys, which I do.
I'll be surprised if I see 50 MPG in time, but that doesn't mean that someone living in Norfolk with a 20 mile flat commute each day couldn't manage nearly 60 MPG.