You will have no problem finding work without the IR and Multi ratings, infact that will make you more attractive to the average flying school as you are not qualified to go anywhere else so they will feel that they can rely on you.
Also at most flying schools (I'm assuming that you want to teach in the UK) they usually pay their part time instructors a higher hourly rate than their full timers!
If you need any more help just post back and I'll see what I can do.
Good Luck!