This is because the oxidation catalyst and diesel particulate filter in the exhaust system have been moved to allow the controls to be switched to the right side of the car. The switch means that the 1.2 litre Polo Bluemotion, which has the lowest CO2 figure of any diesel car on sale in Europe at 87g/km, can't make the same claim in the UK.
The Polo's emissions in the UK are 90g/km, while fuel economy dips from 85.6mpg to 83.1mpg. The two 1.6 litre Polo diesels (74bhp and 89bhp) are also 3g/km worse off in right-hand-drive form, while the official fuel consumption figure falls from 67.3mpg to 65.7mpg. The Polo CO2 emissions in the UK are 112g/km, compared to 109g/km for Mainland Europe.
The good news is that both road tax and company car tax are unaffected by the right-hand-drive car's drop in efficiency – it's not significant enough to move the Polos up a tax band.
Other VWs are not affected by the transformation from left to right-hand drive because there is more bonnet space under the hood to re-route the controls.