At the moment an FI does not need 800 hours IFR to teach for the IR. That is not going to change.
The 800 hours is for an IRI who is someone who is not an FI but wants to teach for the IR.
While I don't expect that the current situation for IMC rating instructors would survive even if the IMC did (the current figure is very low), any proposal to retain the IMC would have to include a propasal for how people can qualify to teach the rating.
Similarly, I expect that the UK factor of 4 system would disappear.
In the end, since training for an IMC rating in the future would only happen in the future at an approved organisation, that organisation would have to set qualification and experience requirements plus set standards for instructors.
You are going to have to take a bet regarding the rating.
However, don't disregard the IR - If you are truly qualified to teach instrument flying and procedures and experienced in actual IMC flight then there is a relatively simple path to the JAA-IR via the FAA-IR if you already have the JAA CPL exams which of course you must have to be an instructor. In other words teaching the IMC rating for a while can help you get an IR with less pain than a non-IMC instructor.
Regards,
DFC