I think there are multiple issues here. Firstly, to have any chance of a prosecution it would be necessary to establish from the top-box manufacturer the reason for the weight limit: it is just as likely to be governed by the load-bearing capacity of the rack to which it is mounted or the motorcycle sub-frame, or it could be the point at which the body of the box distorts and makes locking difficult. Secondly the idea that users routinely weigh their luggage before loading is preposterous, as would weighing a passenger, so the Police would have to have evidence that the rear of the bike was excessively loaded and in an unstable manner. As Andy has already said one extra kilogram is insignificant: if this rider crashed because of losing control he/she would have done so without that kilo, so other factors must be established.