I think there is confusion as to routine check and Roadside vehicle safety check, the police have the right to stop a vehicle on the Queens highway if they suspect that the vehicle has been involved in a crime, has something that makes the vehicle illegal to be used on public roads, they have to show due cause to interrupt the passage of a citizen, this law is open to major interpretation and trust me you would not win in a court of law, they will have (had)valid reason to stop you, once stopped they can then expand their inquiry to other things other than the state of the vehicle I.e. breathalyser, suspicion of carrying drugs,weapons etc. And will ask you to open the boot especially if you start getting barrack room lawry with them as it will only peek their interest as to why you are refusing to open the boot, unless you have committed a crime or have no Tax or insurance plod can not make you leave the car and seize it, they would have to let you go on your way.
But Road side safety checks are a totally different situation when carried out by DVSA/Trained Police in vehicle regulations,
They have horrendous rights in law and can remove your vehicle off the road for any dangerous condition, being over weight on the rear axle (In my opinion most motor homes with a motorcycle in the rear garage are over weight) being one of many reasons they can either seize the vehicle or insist you correct the fault immediately if practical. I.e. Remove the motorcycle to bring the weight into regulation over the back axle or dump fresh water if it will bring the mass weight into regulation, whether you will be fined for being over weight or using the vehicle as a motor home when it should be classed as a Race vehicle with accommodation will be up to the enforcing officer that pulled you into the check area, there is it is said a 5% tolerance but it is not official and will again depend on your attitude and the enforcement officers attitude. There are several places where these checks occur using a DVSA weigh bridge that has been been built just off the road side specifically for weighing commercial vehicles, though they do use weight pads and if you are found to be over a critical weight you will be escorted to a weigh bridge that is used by the DVSA for this reason. Phil