Not strictly true! Whilst there may not be a prosecution for exceeding the "S" value on the grounds of value of the applied load, the authorities could deem the outfit to be unroadworthy The burden of proof is less specific.In a nutshell you cannot be prosecuted due the caravan exceeding its nose weight, but you can be prosecuted for exceeding the towbar specifications or the weight on the rear axle.
Again how accurate would the weighing equipment need to be to detect that either are being exceeded by 20-30kgs?
To be able to measure a value with practical accuracy, the measurement tool needs to be typically a minimum of 10X more precise, and of course be in calibration. However for measuring typical noseload on Class B licence vehicles, it would be perfectly reasonable to expect a measuring device with a range of only 150Kg to be easily be able to resolve to 1kg or even 0.1kg with a tolerance of +/-0.1kg. As far as I know, there is no retail device currently sold which can offer that degree of resolution, and certainly they cannot offer that degree of accuracy. But such systems are readily available in the commercial sector.