also and the
French have been known to prosecute UK drivers for breaking EU rules but not UK rules whilst the vehicle was
in the UK.
How can French prosecute drivers for breaking UK rules????

France can prosecute drivers in France using French rules (or EU rules), they can't use UK rules to prosecute in France!
Simple, the look at your tachograph and irrespective of where you started your journey or ended it, they see speeds in excess of 90kph and fine you... They do this at the roadside and your vehicle does not move 'till it is paid.. You can argue the toss but then you can discuss it at the police station whilst your load is not being delivered.. You do have the option to contest the charges in court but by the time you have done that it works out more expensive than it would if you had paid up and shut up.
The French police also like to see driving times not exceeding 4 hours before a break and also like to see a break of an hour rather than 45 mins.. Again, you can argue but it gets you nowhere and costs more in the long run.. Some large Spanish hauliers have accounts with the French Police to speed up the payment of fines..
So you've never heard of the ECofJ?
Neither has you employer?
Neither have the Spanish hauliers?
Neither have the UK driver's unions?
Nor the Professional Drivers Association?
You talk utter twaddle!
The ECofJ is specifically provided for issues such as this.
Again, provide specific examples of when or where the French police have brought successful prosecutions or fines.
Additionally, you won't have ended your journey, if the French police are able to stop you and examine your tacho'.
So it can only apply to where you started your journey.