In broad terms, the CFI role is focussed on the instructors (instructional standards and standardisation) whereas the HoT responsibilties are focussed on the progress of the trainee and compliance with Part-FCL. As Whopity says, it's all pretty clear in Part-ORA.
All existing EU Regulations have been adopted into UK law meaning that nothing will change after 31 Dec unless and until the existing law is amended by the UK parliament. It is unlikely that there will be any significant change to Part-ORA for some time.