Try to avoid any school that promises you a job just because you do the FI rating there! No school worth its salt will give you a job before they find out what type of instructor you will become and whether they like you!
Think of it the other way around -
Where do you want to work?
Do they do an FI course?
If yes - then there's your answer as many schools who do FI courses prefer their own output for future jobs
If no - then ask your preferred employer where they recommend,
If no recommendation then hunt around for the best you can get.
Depending on locality I would recommend either Caroline at Wycombe Air Centre or Eva at Denham Flight Centre.. Both were orginally trained by Terry Gill who was the "gold standard" in instructing so you can't go wrong!
Other locations - there are plenty of threads on this website with reocmmendations for different places.
As with most things - go see your prospective school and go to the one where you feel confident and comfortable
Good luck!