The way it used to work which I am sure is still the case:
Example: pass ATPL(H) ground school exams and associated flight test and you will have a shiny new CPL(H) licence drop through your door.
The frozen ATPL exam credits are valid for 3 years from either the date of last exam or flight test pass (cannot remember that bit but I thinks it's the former).
If you do your IR within this period then you will have a CPL(H) IR.
You can then spend so many years getting the rest of the requirements for the issue of ATPL(H), i.e MCC, Night etc.
When you have finally got all the requirements you can do the ATPL flight test upgrade and finally your ATPL(H) will drop through the door.
If you do not do your IR within the three years then you will lose the benefit of being able to do it without taking a number of exams (7 possibly) again.
If you don't want to go through the pain of doing many of the exams again then get the IR done within the three years and worry about the rest of the requirements later.
As I say this is how it used to be under JAR.