To teach the IMC you need either an IR(A) for SPA or an IMC rating. You need to be an FI not an FI(R). There is an IFR flight time experience requirement. There is a skill test as well.
Its all here
http://www.caa.co.uk/docs/175/Lasors_Section_H.pdf
Time on the IMC doesn't shorten the JAA IR course although it does count towards the FAA IR
Having an IMC rating will give you a greater weather margin when flying with students, and this might swing a job in your favour.