On most courses the madatory Instrument Flying section can be reduced IF the student already holds an IMC. Only applies to UK FTO's of course. IMC training can be had at circa 80 pounds per hour whereas CPL training often weighs in around the 200 pounds per hour mark. Big difference.
Not sure about the comments about not using an IMC when an instructor.
IF you become a FI prior to your IR (which I recommend you do as its easier to pass an IR with 1000hrs instructing under your belt) you need SOME way of flying IMC. The only way for you to do that is to hold an IMC rating yourself...
You'll be a poor FI if you have to sit on the deck all day whilst colleagues blat off into the stratus to do some beacon bashing/VFR on top.
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