A few notes:

The "rate of employed" you quoted is usually for 15-64, you seem to imply that the higher the better. Hence having 100% of 15-24 work full time (and no study) is better in the long run for the economy? Right? Having housing workers move weights from 55 to 64? So you have to be careful. Second point as you note part time is 25% in the UK and 15% in France, this is not shown by the statistic you used.

About hours worked the statistics you cite comes from OECD however if you look at the OECD document there's a big warning in the first page of the document: hours worked as shown by OECD are not comparable between countries (for example extra work hours are counted in for the UK but not counted for France). They say time serie is useful for trend but I doubt that fact because of the source of the measure.

On the solution part there's a lot to say but if you're looking at the wrong measure you're unlikely to come up with working solutions.