No I can't, but I have always assumed that was because passenger train weights make a Bo-Bo configuration acceptable.
Also, modern electronic engineering allows, in North America, the current designs to have A-1-A bogies, where the middle axle simply spreads to weight, but the engines are 4,400hp diesels where the power is delivered by two axles per bogie. In the British context 3,300-ish hp is enough, but why can't that be delivered by a Bo-Bo?
I might be over simplifying here, but that is my understanding.