Their motorways have different tarmac that makes less noise/provides slightly less rolling resistance so you'll get better economy.
I ALWAYS find my diesel cars do better MPG on the continent, partly because we are only ever doing long distances, multiple tanks of motorway work on cruise control with no town/country road driving, partly maybe heat, in summer its often warmer on the continent, diesels prefer being warm. But I think fuel quality has got something to do with it as well. In Germany/Austria/Hungary we often get 'truck' diesel, which I think has a lower cetane rating (bad), but somehow we get better mpg from it. Might be it has a lubricant in it that our mechanical fuel pump engined car likes more? Smell from exhaust is defo MORE dieselly on euro truck stop fuel.
Prevailing wind as well? Makes a huge difference even 15mph of wind tail/head wind can make a big swing in your MPG figures.
Water in fuel, could be you had some milky crappy water logged fuel that you've been using when in UK, then you've done several big full tanks of fresh fuel and used up all the water laden fuel (I get big issues with this on motorbikes, lots of water in UK fuels IME).
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