The UK CAA do take action to discourage dubious public transport flights but must use their resources carefully, the result of this is that the inspectors turn up at places that are likely to be hot spots for this sort of activity.
I have been ramp checked twice on the Isle of Man during the TT races, these checks clearly looked like them going through the motions while waiting for their real targets to show up.
It is hardly surprising that they can’t show up at Nantes on a January day in the hope of catching one person with the resouces available to them but I suspect that there is more intelligence targeted activity than you might think. It is also likely that a word or to with people who have one foot ether side of the line stops illegal activity without going to court.
The trouble is there are always a few chancers who will step up when there is a few quid to be made and those who will cut corners on equipment and pilot training to increase that Proffit. Unfortunately the well meaning cost sharing legislation has further blurred the lines of these activities making the job of the CAA harder.