If they take officially paid hours it can change the picture.

Also GDP is price-based, whereas some productivity numbers you see might be based on physical output (in the manufacturing sector for example).

http://en.wikipedia.org/wiki/Labour_productivity

No economist will admit that total hours really worked is similar in France and in the UK to start with: that's contrary to orthodox economics since UK is liberal with a flexible labour market and France is full of rigidity and must be reformed, so the only way to make number match is to say that french worker are more productive per hour.

And I'm pretty sure 99.999% of economists haven't the slightest idea on how the productivity numbers they cite all day long are really compute and what they really mean.