To teach IMC you only need 10 hours IF. The words that say this are well hidden and even the CAA can struggle to find the correct legislation sometimes. You only need the 200 hours to teach the IR.
Do the rating, its great. You get to fly on poor days and your own IF gets much better. The only downside I found of having the IMC inst rating is you miss out on the trips to the greasy spoon when its raining.
For a bit of spice go and do the Aeros instructor course as well.