Your intial IR isn't counted as a skills test on the MEP class.
And doing the CPL in a SEP is a pain in the ring if your used to flying twins.
Speak to your school about it. There is bound to be an examiner kicking about and you might find that they want to do a couple of hours nav with you in a SEP. You should be able to get it sorted without to much hassel and expense.
Doing everything in the twin isn't cost effective. Some of the CPL stuff is quite anal retentive with paper work, timings and RT. Better to get that squared away paying 150 quid an hour for the aircraft than 300 plus.