I'm not aware of any European Standard for bicycle saddles, only one for child seats; EN14344 European Standard for Child's Seats for Bicycles. Category C15 within the standard is seats to be mounted between the handlebar and the rider and that can carry children up to 15 kg. If you can find another European standard that covers saddles generally I might revise my assumptions but absent that its difficult to see what other interpretation is possible.
The fact that such seats are specifically covered in the Standard means they are officially accepted to be safe and an acceptable modification to a bike to carry a child passenger whatever some uninformed bobby might think
As for pleading guilty it could easily be ignorance of the law (he defended himself) or pragmatism in the face of the (generally much more expensive) alternative. The system encourages admission of guilt especially if you are not well off. If you contest an FPN you go to Court and get, as here, a much higher penalty even if you plead guilty and if you plead not guilty you generally get hit by a much higher penalty still. Remember Daniel Cadden would have had to accept the guilty finding of a magistrate had the CTC not come forward to pay to contest it. That finding initially cost him £300 for failing to cycle in a cycle lane.
