Hi FN,
Basically if you apply for CPL, ME rating and Initial IR at the same time you get three for the price of one i.e £194.
Your Cpl skills test and ME test are valid for 12 months so if you are going to do your IR immediately after your CPL it makes sense to wait and apply for them together.
Also by applying for ME and IR together you can do your annual Renewal at the same time.
regards
Willby