Its difficult to buy cranberry juice, what you are most likely buying is Cranberry juice drink, which is not the same at all !!!!
if the description says Juice drink it can be made up of a low % of the juice and the rest made up with water, sugar and flavourings and colouriings!
if you want the health benefits look for the word Juice not Juice drink! this applies to Orange, Apple and all other juices sold in the UK.