To teach instrument flying, ie the IMC rating or the IR.
When you get an FI(A) rating, it comes with a "no applied instruments" restriction, which you need 200 hours IFR to be eligible to remove.
The CAA used to do a nice practical solution and say that 1 hour flying in actual IMC, so fully on instruments, would equate to 4 hours flying IFR (where you could be in VMC the whole time).
So that meant if you had logged 50 hours actual instrument time, you could get the restriction removed.
Mad_jock is referring to the fact the CAA removed this nice solution, so now you need 200 hours IFR, which very few instructors will be able to build up to, so there will definitely be a shortage of instructors able to teach this rating.