Sarakelly1234, sorry for my ignorance but I have no idea what or where "austro" is. You have a UK PPL licence and medical and wish to instruct within the UK. Why are you thinking of changing your licence and medical away from the UK? If you are doing the ATPL exams in order to instruct but do not intend to be an airline pilot, which you don't mention, then there is no need to do your ATPLs. For the PPL Instructor Rating you only need exams at CPL level. If you do the commercial exams ATPL/CPL, as you choose, in the UK that will be the simplest way. Same for the Instructor course.
Forget brexit at the moment, it is not helpful to speculate on that. People are over complicating things by bringing it into every discussion.