Hi James,
Unfortunately this will not be possible, this is a known Jaspersoft limitation:
Clarity cannot pass en_GB as locale to Jaspersoft, since it will only accept en (or en_US). So US format will be used. Our developers are currently working on workarounding this limitation. It was logged as CLRT-78444 for your future reference.
Kind Regards
Nika Hadzhikidi
CA Technologies
Principal Support Engineer