You can let both MEP and IR lapse by up to five years without any problem whatsoever. Renewal can be with any duly authorised examiner without the CAA getting involved. Go over the 5 year deadline and things get more complicated, but my guess is you are not planning to let it get that far.
One thing to remember is that when somebody is hiring the applicant with a current IR might well be in a better position than the applicant without one.