The Radiator acts as an exchange point where air is blown between the radiator fins that house the coolant causing the air to cool the coolant. It is an integral part of any watercooling loop and the better the radiator the more efficient it is at getting rid of heat from the loop and therefore heat from the components it cools. In true UK Gaming Computers style we use the best of the best Hardware Labs radiators rather than some unknown brand - the last thing you want is your system to spring a leak or overheat from a poorly chosen radiator. We also ensure each radiator we use is flushed 4 times with a vinegar solution followed up by 4 distilled water passes to ensure a completely clean loop prior to filling.
Radiators are pretty big and will only fit in certain cases so usually it is not possible to upgrade the radiator for each water cooled PC but rest assured each radiator chosen by us is more than capable of the task in hand.