The simple answer to this question is a GVW (Gross Vehicle Weight) of 3.5 tonnes. However, if you passed your test before 1st January 1997 then technically you can drive up to a GVW of 7.5 tonnes for non-commercially purposes (Currently we do not have 7.5GVW vehicles for hire).
So what is GVW? Gross vehicle weight is the total weight of the vehicle including load, passengers & fuel. If you exceed the maximum GVW of a vehicle then you are breaking the law and the penalty can be significant. Not to mention that it will impact the safety of the vehicle.
There is a page on the Government website with a clear and concise description of GVW here.
All of our hire vehicles are 3.5 tonne GVW or less. If you have any doubts about how much you can carry you need to find out the GVW of the vehicle and then use a weighbridge if necessary. The driver of the vehicle is solely responsible for the vehicle and weight.