Many schools that run FI courses take students from their courses on as instructors. At the school I did mine at I know of several people who have been offered jobs on completion of the course.
I myself got a part time job instructing at the club I had been a member of since I started flying. My rating arrived on a Thursday, got the call on the Friday, started on the Saturday!
Go and do the course is what I say. Best course I have ever done in the aviation world. There seems to be plenty of FI work out there in the south east at the moment if you go looking.
Even if you only get a part time weekend job the hours soon add up. It can be very rewarding as you see your students develop and there is the added bonus you no longer have to pay to go flying
