Clarification received yesterday from the UK CAA:
At the time of writing this email, if the UK is to be the state of licence issue, it has been agreed* to allow PTC students, who have partially complete their ATPL exams, to sit the outstanding exams with a UK ATO, once all exams have been completed, within the validity periods, the UK CAA will accept the exam results from the IAA and the UK CAA towards licence issue, ensuring all requirements of FCL.025 are satisfied.*