The big problem with hybrid vehicles with plug-in capability is that the real world mileage and emissions are highly dependent both on duty cycle and the ability to actually plug in. The Vauxhall Ampera (aka the European Chevy Volt) is expected to be rated at approximately 40 g/km of CO2 emissions. That's based on the fact that with a full charge it can complete 40 miles of the 60 mile EU test cycle on electric power alone. Those who are able to actually plug in their vehicles every day will very likely be able to achieve something close to this. But therein lies the rub. Not everyone can plug in a vehicle.
In European cities in particular, many people don't have a garage that they can put the car into and plug it in. Curbside charging is also extremely limited. As a result, many drivers will find that their mileage is significantly lower than those who plug in all the time. This problem is, of course, not unique to Europe nor the Volt/Ampera, but the layout of population centers in the UK and elsewhere may exacerbate it compared to the U.S. GM evidently has no plans to adjust the battery usage cycle to accommodate less frequent charging since that would negatively impact the battery lifespan.