The French work statistics count the lunch hour (or two hours) as part of the working day. I kid you not.
So comparing UK data with that from France is like comparing apples with pears.
Not true, standard working week in France is 35 hours and for every six hours worked there has to be a minimum 20 minute UNPAID break.
As in Spain the overall work day in France is often counted as part of welfare and work / life balance statistics, which includes commuting and lunch breaks. In that way you can look at overall productivity from a complete work day, but you could do the exact same thing in the UK. You can then compare apples with apples and pears with pears.
Where the problem exists is in the practice of who counts their hours and who doesn't. Very few professionals clock on, but many unskilled and semi skilled and many government workers do have to clock on and off. Typically many professional workers have to log their hours worked in order to be billable. But even if lets say a lawyer or a consultant was clocking 50 or 60 billable hours in a week, on typical productivity stats it would only show a salaried member of staff on a full time 35 hour per week contract. Internal productivity would look completely different.
Spain recently introduced legislation where all employees have to clock in and out, in order to combat over working (for free) what this has done though is encouraged large companies to have their staff work at home where due to a privacy loophole in the law they don't have to record those hours.
The point I'm making goes to the issue that productivity stats are completely pointless when you get to professional / service orientated work, and impossible to calculate when you look at the majority of service work now not even having a productive output, the UK is now a majority Rentier economy (according to Paul Krugman and others), and productivity in that instance can only be measured in asset value increase and yields, and asset values generally are not even included as a calculated item on GDP, which GDP per capita, the base measure of productivity can not make any sense.
edit: just to add if the lunch breaks were included as you say it would make the french productivity look even better. so on top of the tax increases, worker protections et al that Truss(ed up in daft ideas) wants to eradicate there's a decent case for paid 2 hour lunch breaks