Point 1 - The general premise is that you cannot instruct for any licence or rating that you do not hold yourself.
Point 2 - All UK requirements for FI's are now aligned with JAR-FCL.
To instruct for the issue of a IR(A) you will need:
An FI Rating.
An Instrument Rating.
At least 200 hours flight time in accordance with the instrument flight rules, of which up to 50 hours may be instrument ground time in a flight simulator or FNPT 2.
Complete an approved course comprising at least 5 hours of flight instruction in an aeroplane, flight simulator or FNPT 2.
Pass a Skill Test with an FIE.
See JAR-FCL 1.330(d)