Don't follow the reasoning that the enemy treats prisoners with barbarity so, logically, we should do the same to their prisoners, to even things up a bit. The Japanese were not above treating prisoners with barbarity, but we did not see fit to treat Japanese prisoners the same way. We treat them fairly because that is morally right and, you may think, it only hands the enemy a propaganda coup if we abandon our principles.
Even the Nazis had some principles (though you may correctly feel that these were sometimes forgotten). There was one reported incident of German soldier urinating on a French war memorial. He was promptly shot dead by an officer. That was for showing lack of respect for dead soldiers, though they had died fighting the Germans in the previous World War. Not exactly the British way, but principled. That the Germans once strung up a farmer, in front of his family, because he was heard making a disparaging remark as the Germans retreated, does not alter that.