UV Disinfection, Residual Chlorine Dose and the Contact Tank

In my organization there are some treatment plants with UV disinfection + residual chlorine dose + contact tank.

Lately it has been questioned whether the presence of the contact tank is really needed.

In terms of being compliant with UK legislation, there is no requirement to have a contact tank after UV, as the UV and residual dose are sufficient.

However, the risk of pathogenic contamination would effectively increase by removing the contact tank after. 

Is a contact tank after UV and residual chlorine dose necessary?

What is common practice in other organization and is it usual or not to have this arrangement of UV + Residual with contact tank after?