This surely comes down to the GVW of the vehicle, and also safe loading procedures with the empty skips. Drivers responsibility to safely load his/her vehicle and stay within the vehicles weight limits.

Is this was you are looking for: http://chem.uk.com/index.../86-vosa-policy-guidance Skips Empty or loaded skips may be carried either on dedicated skip lorries or on flatbed vehicles. It is recommended that skips are carried on skip lorries wherever possible, as it can be very difficult to adequately secure a skip on a flatbed vehicle. Loaded or partly-loaded skips should not be stacked on top of each other for transport, even on a dedicated skip lorry. The lower skip does not provide a stable base and there is a risk of the upper skip/s moving under sudden braking, or falling from the side of the vehicle under a combined steering and braking manoeuvre (for example, swerving to avoid another road user). Some skip lorries have a shaft connecting the upper ends of the lifting arms and this is used by some operators to “press down” on stacked skips. This is not recommended, as the use of the lifting arms for load restraint can lead to fatigue cracking in the lifting arms. Empty skips can be stacked up to three high without further restraint as long as they are securely ‘nested’.