Currently there is an exemption in place which means that UK IR holders do not need PBN endorsement unless they wish to fly PBN approaches or in airspace which mandates <RNAV5.
Another exemption applies to commercial operators who do not use PBN procedures if their aircraft are not fitted with PBN-compliant systems.
From 2020, all civil IRs must include PBN endorsement; this will be achieved through training and PBN testing during initial IR Skill Tests or revalidation proficiency checks.
It should be fairly simple to accommodate PBN training/testing for military IRs - it's hardly a big deal.